A method of forming a semiconductor structure includes following steps. A first isolation is formed between a pair of active regions. A gate structure is formed on the first isolation structure. The active regions are etched to form recesses with curved top surfaces. The active regions are etched again to change each of the curved top surfaces to be a top surface and a sidewall substantially perpendicular to the top surface. A pair of contacts is formed respectively on the active regions, such that each of the contacts has a bottom surface and a sidewall substantially perpendicular to the bottom surface.