A CMOS transistor manufacturing method includes: forming a gate insulating film on a semiconductor substrate; forming a first gate electrode pattern on the gate insulating film in an NMOS transistor area; forming a second gate electrode pattern on the gate insulating film in a PMOS transistor area; forming a first photoresist pattern covering the NMOS transistor area to expose the second gate electrode pattern; performing a first ion injection process into the PMOS transistor area to form an n-type well region and a p-type LDD region; removing the first photoresist pattern; forming a second photoresist pattern covering the PMOS transistor area to expose the first gate electrode pattern; performing a second ion injection process into the NMOS transistor area to form a p-type well region and an n-type LDD region; removing the second photoresist pattern; and forming sidewall spacers at sidewalls of the first and second gate electrode patterns.