can i bake a cake using canola oil instead of butter?!

You can use apple sauce also. It tastes great in cake and it will work the same as oil.
If you use oil, be careful of the taste you are adding - like Olive Oil is pretty strong tasting.

Choose a recipe that calls for vegetable oil instead. Oil is NOT a substitute for butter because of heat and consitancy and most of all...taste. Butter packs a lot of flavour. Recipes that call for oil bear this in mind and crank up the sugar or things like cinnamon or cocoa.

You could, but the flavor and the texture will be different. I wouldn't.