A method of fabricating semiconductor device is provided. First, a recess having a substantially rectangular cross section is formed in a substrate. Then, oxide layers are formed on sidewalls and bottom of the recess by oxygen ion implantation process, wherein oxide layer on sidewalls of recess is thinner than oxide layer on bottom of recess. Thereafter, oxide layer on sidewalls of recess is completely removed, and only a portion of oxide layer on bottom of recess remains. Then, sidewalls of recess are shaped into Σ form by orientation selective wet etching using oxide layer remained on bottom of recess as a stop layer. Finally, oxide layer on bottom of recess is removed. By forming oxide layer on bottom of recess and using it as stop layer in subsequent orientation selective wet etching, the disclosed method can prevent a Σ-shaped recess with a cuspate bottom.