A semiconductor device includes a gate structure on a substrate, a first spacer on sidewalls of gate structure, a second spacer on sidewalls of the first spacer, a polymer block adjacent to the first spacer and on a corner between the gate structure and the substrate, an interfacial layer under the polymer block, and a source/drain region adjacent to two sides of the first spacer. Preferably, the polymer block is surrounded by the first spacer, the interfacial layer, and the second spacer.