Disclosed is a semiconductor device including an oxide semiconductor film. A first oxide semiconductor film with a thickness of greater than or equal to 2 nm and less than or equal to 15 nm is formed over a gate insulating layer. First heat treatment is performed so that crystal growth from a surface of the first oxide semiconductor film to the inside thereof is caused, whereby a first crystal layer is formed. A second oxide semiconductor film with a thickness greater than that of the first oxide semiconductor film is formed over the first crystal layer. Second heat treatment is performed so that crystal growth from the first crystal layer to a surface of the second oxide semiconductor film is caused, whereby a second crystal layer is formed. Further, oxygen doping treatment is performed on the second crystal layer.