Does Ontario charge its own corporate tax on top of federal tax?

Yes. Canadian corporations pay both federal income tax and provincial income tax. Ontario administers its own corporate income tax, and the two rates are stacked to produce the combined rate you actually pay. Tax is largely a federal matter under the Income Tax Act, but provinces set their own rates within that framework.

For a qualifying CCPC earning active business income up to the annual small business limit, the combined federal plus Ontario rate is meaningfully lower than the general rate. For income above the limit, or income that does not qualify for the small business deduction, the combined general corporate rate applies. Investment income earned inside a private corporation is subject to a separate, higher tax regime designed to prevent using a corporation as a tax shelter for passive income.

These rates change from time to time through federal and Ontario budgets, so always confirm the current figures with your accountant. The important planning point is that both levels of government are collecting tax, and Ontario's rate matters when you model your after-tax retention inside the corporation.