A method of manufacturing a semiconductor device includes forming a first mask pattern exposing a first region for forming a first transistor and a second region for forming a second transistor, performing a first ion implantation for forming well regions using the first mask pattern, performing a second ion implantation for threshold voltage (Vth) adjustment of the first transistor using the first mask pattern, removing the first mask pattern and forming a second mask pattern in which the first region is covered and the second region is opened, performing a third ion implantation for Vth adjustment of the second transistor using the second mask pattern, forming first and second gate insulating films in the first and second regions respectively, and forming first and second gate electrodes in the first and second regions respectively.