I have a question, if someone (Grandparents) are French passport holder, and then migrate to overseas and give birth overseas (Either father or mother), and then either father or mother give birth overseas again, do this dependent eligible to apply French passport or not?

Grandparents - French passport - born in France
Parents (Either father or mother) - French passport - born in overseas
someone - born in overseas

Thank you

Dans quel cas un enfant est-il français ? - Service-public.fr
If one of the parents had French nationality at the time the child was born, then the child has French nationality.
Seems simple enough.

Hi, ET has found the link confirming "filiation" for France, but I'm intrigued that you've posted the same thread on the Portugal forum. Are the two grandparents French and Portuguese? and their child has dual nationality? and he/she would like their child to be either French or Portuguese?
If no one replies on the Portuguese forum, you'd do best to consult the Portuguese consulate where the grandchild was born....
Be warned that there's a lot of paperwork involved with justifying the grandchild's eligibility for French nationality, particularly grandparents' and parents' birth certificates. The grandparents' BCs have to be applied for at the Mairie of their respective place of birth.... Hope this helps!

Also, be aware that whilst having a French/Portuguese/EU passport can potentially make life easier in some ways, it doesn't in itself give you many privileges in practical terms. EU citizens only acquire rights and privileges in a country where they've lived and worked and contributed. In an EU state where you've never worked or contributed, you don't really have any more rights than a non-EU citizen, beyond the fact that you can come and go and apply for jobs without needing to apply for visas.
I'm just mentioning that because quite often on forums you see people who are under the misapprehension that if you have say a French passport, if you come to France you'll automatically entitled to French social security and so on, and are disappointed when they discover that that's not the case.

Agree.
For example, if they decided to live in the UK, my Anglo-French daughters, with dual nationality, would have to contribute to the UK NHS if they wanted to be covered for healthcare.
An EU Passport allows movement within the EU, but does not guarantee work or healthcare coverage.

You are so smart, france + portugal, it's my friend and he is born in Australia, and his mother and father also born in Australia, so he want to know he can get France / Portugal passport or not? Thank you.

Like it says in the link - if one of his parents had French citizenship when he was born, then it looks as if he should be able to apply for a French passport.
I haven't a clue what Portugal's ruling on this is.
If he lives in Australia, I would suggest he contacts the French and/or Portuguese embassy in Australia to ask what the process is.

The link certainly says so, but maybe immigrants doesn't understand French?
As you suggest, the friend should contact either or both Embassies in Australia, to ask directly.
But I repeat that, unless his French relatives were all born in the same town, he's going to have to apply to several French Mairies (Town Halls) to obtain their Birth Certificates, and get them sent snailmail to Australia. Probably Marriage Certificates, too. If he posts directly on BE, we can give more info as to how to go about it. Does he speak French in order to fill in the form on-line?
But best to contact his nearest French Embassy first, for a complete list of necessary documents. Likewise for the Portuguese side of things. British Expats in France and Portugal aren't the best placed to advise on this and can only point people in the right direction, i.e. the above link.

Let me tell him, thank you

It sounds like a nice idea to maintain connectons with the country of your ancestors, but France still has some form of national service which applies to girls as well as boys

The year book of our commune states that youngsters over a certain age need to register their address with the Mairie.

Is this only for "French" people who are resident or for all residents in france or for all French people including those visiting or on holiday/

Recensement militaire (ou recensement citoyen) - Service-public.fr

My two daughters did their "National Service" while at Lycée. It might have changed since then, but it consisted of a day when they were made aware of civic responsibilities and, quite honestly, it was a waste of time as they'd already learned all about them in Collège.
Youngsters with French ID card have to register their address with the Mairie the year that they are 18, in order to get on the Electoral Roll and receive their Carte d'Electeur. And all French citizens over 18 register at their new Mairie if they move, in order to be able to vote there. Those who live or work abroad register at the local French Consulate. ("Voting is a civic right and duty", as learned above).
P.S. Just seen ET's link. I don't recollect my daughters doing the recensement thing at 16, but things must have changed since their day.

This comment interested me, as I was pretty sure that the 'real' military service was no longer mandatory since 1988/89. I recall our son just missed it by six months, and was somewhat relieved as a result.
Reading the link provided by ET, it appears that a one day citizens military awareness attendance is required by persons with French nationality, who thereafter may participate if they wish in any national examination.