Canada continues the Westminster tradition of using the title Prime Minister when one is speaking to the federal head of government directly; the Department of Canadian Heritage advises that it is incorrect to use the term Mr. Prime Minister.
The written form of address for the prime minister should use his or her full parliamentary title: The Right Honourable [name], [post-nominal letters], Prime Minister of Canada.
However, while in the House of Commons during Question Period, other members of parliament may address the prime minister as the Right Honourable Member for [prime minister's riding] or simply the Right Honourable Prime Minister.
Former prime ministers retain the prefix the Right Honourable for the remainder of their lives; should they remain sitting MPs, they may be referred as the Right Honourable Member for [member's riding], by their portfolio title (if appointed to one), as in the Right Honourable Minister of National Defence, or should they become opposition leader, as the Right Honourable Leader of the Opposition.

In the decades following Confederation, it was common practice to refer to the prime minister as Premier of Canada, a custom that continued until the First World War, around the time of Robert Borden's premiership.
While contemporary sources will still speak of early prime ministers of Canada as premier, the modern practice is such that the federal head of government is known almost exclusively as the prime minister, while the provincial and territorial heads of government are termed premiers (in French, premiers are addressed as premier ministre du [province], literally translated as prime minister of [province]).
Canada continues the Westminster tradition of using the title Prime Minister when one is speaking to the federal head of government directly; the Department of Canadian Heritage advises that it is incorrect to use the term Mr. Prime Minister. The written form of address for the prime minister should use his or her full parliamentary title: The Right Honourable [name], [post-nominal letters], Prime Minister of Canada. However, while in the House of Commons during Question Period, other members of parliament may address the prime minister as the Right Honourable Member for [prime minister's riding] or simply the Right Honourable Prime Minister. Former prime ministers retain the prefix the Right Honourable for the remainder of their lives; should they remain sitting MPs, they may be referred as the Right Honourable Member for [member's riding], by their portfolio title (if appointed to one), as in the Right Honourable Minister of National Defence, or should they become opposition leader, as the Right Honourable Leader of the Opposition.
In the decades following Confederation, it was common practice to refer to the prime minister as Premier of Canada, a custom that continued until the First World War, around the time of Robert Borden's premiership. While contemporary sources will still speak of early prime ministers of Canada as premier, the modern practice is such that the federal head of government is known almost exclusively as the prime minister, while the provincial and territorial heads of government are termed premiers (in French, premiers are addressed as premier ministre du [province], literally translated as prime minister of [province]).
Prime Minister of Canada