An object is to provide a semiconductor device including an oxide semiconductor film, which has stable electrical characteristics and high reliability. A stack of first and second material films is formed by forming the first material film (a film having a hexagonal crystal structure) having a thickness of 1 nm to 10 nm over an insulating surface and forming the second material film having a hexagonal crystal structure (a crystalline oxide semiconductor film) using the first material film as a nucleus. As the first material film, a material film having a wurtzite crystal structure (e.g., gallium nitride or aluminum nitride) or a material film having a corundum crystal structure (α-Al2O3, α-Ga2O3, In2O3, Ti2O3, V2O3, Cr2O3, or α-Fe2O3) is used.