CALGARY—Athabasca Oil Sands Corp. said it was selling a 40% stake in one of its oil-sands prospects to PetroChina Co., a move that for the first time will give full ownership of such a project to a Chinese company.

Athabasca is selling its remaining interest in the MacKay River project in northern Alberta to PetroChina for 680 million Canadian dollars, or US$666 million. In 2010, Athabasca sold 60% stakes in MacKay and a separate development, Dover, to PetroChina for C$1.9 billion.

Chinese...