A semiconductor device including a first gate structure and a second gate structure immediately adjacent to each other with a spacer therebetween. Line width of the top of the second gate structure is not less than that of the bottom thereof. A fabrication method thereof is also disclosed. A transient first gate structure and a temporary gate structure are formed by etching through a first hard mask. A second gate structure is formed between a first spacer and a second spacer opposite to each other and disposed respectively on the transient first gate structure and temporary gate structure. The second gate structure is covered with a second hard mask. An etch process is performed through a patterned photoresist layer to remove exposed first hard mask and temporary gate structure and to partially remove exposed portion of first hard mask and transient first gate structure to form the first gate structure.