In Canada, drinking and driving is always a felony offense. As such, if you have a conviction for DUI in another country, they view that as a felony also (regardless of what it is in the country where commited).
That being said, it is up to the border agent where you cross if they will let you in or not. Sometimes they will, sometimes they won't.
You can also apply prior to your trip for permission to enter Canada with a prior DUI, but it costs a lot and takes months to complete.
Eventually (I believe 10yrs, but could be wrong) you will allowed in hassle free.