Yes, as long as you have US Citizenship by virtue of birth and not naturalization. If you are a Naturalized US Citizen and were a Canadian Citizen before becoming a US Citizen you are suppose to give up your Canadian citizenship. Although, no one actually does.
If you are a Naturalized US Citizen and then became a Naturalized Canadian citizen you would lose your US Citizenship, since Naturalized US Citizens cannot become naturalized citizens of other countries without losing US Citizenship.
Even if you did this, again, unless you have somehow annoyed the US Government, you most likely would not automatically lose your US citizenship. For this to happen, either you would have to tell the US Government you have become naturalized in another country or Canada would have to tell the US you have became naturalized in Canada.
Basically, if you were born in the US or one of your parents was a US citizens at the time of your birth, your citizenship can never be taken away from you.
The US does not officially recognize dual citizenship. This does not mean another country cannot determine you to be a citizen of said country, but as far the US is concerned you are either a US Citizen or you are not a US Citizen. Any other citizenship is between you and the country that claims you as a citizen.
