A silicidation process for a MOS transistor and a resulting transistor structure are described. The MOS transistor includes a silicon substrate, a gate dielectric layer, a silicon gate, a cap layer on the silicon gate, a spacer on the sidewalls of the silicon gate and the cap layer, and S/D regions in the substrate beside the silicon gate. The process includes forming a metal silicide layer on the S/D regions, utilizing plasma of a reactive gas to react a surface layer of the metal silicide layer into a passivation layer, removing the cap layer and then reacting the silicon gate into a fully silicided gate.