Provided are a semiconductor device including a high voltage transistor and a low voltage transistor and a method of manufacturing the same. The semiconductor device includes a semiconductor substrate including a high voltage region and a low voltage region; a high voltage transistor formed in the high voltage region and including a first active region, a first source/drain region, a first gate insulating layer, and a first gate electrode; and a low voltage transistor formed in the low voltage region and including a second active region, a second source/drain region, a second gate insulating layer, and a second gate electrode. The second source/drain region has a smaller thickness than a thickness of the first source/drain region.