An embodiment of a photomask for forming gate lines and a method of manufacturing semiconductor devices using the photomask is disclosed. The photomask includes a photomask substrate, gate line mask patterns that define gate lines that cross at least one active region on a semiconductor substrate, and that are arranged in parallel, gate tab mask patterns formed on both sides of each gate line mask pattern, and joints formed between adjacent gate tab mask patterns, and that include a separation region. A relatively large gate tab mask pattern can be formed using the photomask. And a short channel effect at the boundary of the active region can be improved with the large gate tab mask pattern, so the characteristics and reliability of the semiconductor devices can be improved.