From Wiki

If you have been convicted of a criminal offence, you will need to apply for Criminal Rehabilitation before you can apply to enter Canada.

Citizenship and Immigration Canada (CIC) explains the process on its web page entitled Rehabilitation for Persons Who are Inadmissible to Canada Because of Past Criminal Activity.

The extent to which this delays entry to Canada varies from person to person. Some people have found it has delayed them about a year, while others have found it has delayed them by as little as ten weeks.