The invention discloses a method of forming a metal-oxide-semiconductor transistor. The method provides a substrate, where a gate structure is formed thereon. Next, a first spacer is formed on the sidewall of the gate structure. A pair of heavily doped regions is formed in the substrate. Then, an annealing process is performed to make the doped ions in the heavily doped regions uniformly distributed. Next, the first spacer is removed and a thin pad dielectric layer is formed over the substrate. Next, a first type halo structure is formed in the bottom portion of the source/drain region beneath the gate structure. A lightly doped region is formed between the gate structure and the first type halo structure and above the first type halo structure. An etching process is performed on the pad dielectric layer to form a second spacer and then the MOS transitor is completed.