A semiconductor element having high mobility, which includes an oxide semiconductor layer having crystallinity, is provided. The oxide semiconductor layer includes a stacked-layer structure of a first oxide semiconductor film and a second oxide semiconductor film having a wider band gap than the first oxide semiconductor film, which is in contact with the first oxide semiconductor film. Thus, a channel region is formed in part of the first oxide semiconductor film (that is, in an oxide semiconductor film having a smaller band gap) which is in the vicinity of an interface with the second oxide semiconductor film. Further, dangling bonds in the first oxide semiconductor film and the second oxide semiconductor film are bonded to each other at the interface therebetween. Accordingly, a decrease in mobility resulting from an electron trap or the like due to dangling bonds can be reduced in the channel region.