A method of forming a MOS transistor, in which a co-implantation is performed to implant an implant into a source region and a drain region or a halo implanted region to effectively prevent dopants from over diffusion in the source region and the drain region or the halo implanted region, for obtaining a good junction profile and improving short channel effect. The implant comprises carbon, a hydrocarbon, or a derivative of the hydrocarbon, such as one selected from a group consisting of C, CxHy+, and (CxHy)n+, wherein x is a number of 1 to 10, y is a number of 4 to 20, and n is a number of 1 to 1000.