Laser transformation hardening(LTH) was applied to the surface of the AISI 420 martensitic stainless steel by a pulsed Nd:YAG laser to obtain optimum hardness.The influences of process para-meters(laser pulse energy,duration time,and travel speed) on the depth and hardness of laser treated area were investigated.Image analysis of SEM micros-tructure of AISI 420 showed that plate-like carbide have almost fully and(30~40)% of globular carbide particles dissolved into the matrix after laser transformation hardening by pulsed laser and the microstructure was refined to obtain controlled tempered martensite microstructure with 450 VHN hardness. Laser transformation hardening (LTH) was applied to the surface of the AISI 420 martensitic stainless steel by a pulsed Nd: YAG laser to obtain optimum hardness. These influences of process para-meters (laser pulse energy, duration time, and travel speed) on the depth and hardness of the laser treated area were investigated. Image analysis of SEM micros-tructure of AISI 420 showed that plate-like carbide was almost fully and (30-40)% of globular carbide particles dissolved into the matrix after laser transformation hardening by pulsed laser and the microstructure was refined to obtain controlled tempered martensite microstructure with 450 VHN hardness.