The invention provides a method for forming a transistor, which includes: providing a substrate, a semiconductor layer being formed on the substrate; forming a dummy gate structure on the semiconductor layer; forming a source region and a drain region in the substrate and the semiconductor layer and at opposite sides of the dummy gate structure; forming an interlayer dielectric layer on the semiconductor layer; removing the dummy gate structure for forming an opening in the interlayer dielectric layer; non-crystallizing the semiconductor layer exposed in the opening for forming a channel layer; annealing the channel layer so that the channel layer and the substrate have same crystal orientation; and forming a metal gate structure in the opening, the metal gate being formed on the channel layer. Saturation current of the transistor is raised, and the performance of a semiconductor device is promoted.