An object is to provide a method for manufacturing a thin film transistor having favorable electric characteristics, with high productivity. A gate electrode is formed over a substrate and a gate insulating layer is formed over the gate electrode. A first semiconductor layer is formed over the gate insulating layer by generating plasma using a deposition gas containing silicon or germanium, hydrogen, and a rare gas. Next, a second semiconductor layer including an amorphous semiconductor and a microcrystal semiconductor is formed in such a manner that the first semiconductor layer is partially grown as a seed crystal by generating plasma using a deposition gas containing silicon or germanium, hydrogen, and a gas containing nitrogen. Then, a semiconductor layer to which an impurity imparting one conductivity is added is formed and a conductive film is formed. Thus, a thin film transistor is manufactured.