SCM440, which is widely used as the diesel engine piston of vessel, has been hardened by a CO₂laser with the wavelength of 10.6㎛. Laser hardening experiment has been carried out for the condition of a laser power 1㎾, the travel speed between 0.4 and 1.5m/min, and a rectangular-Gaussian beam. Residual stress has been measured by using middle point technique of half value width of X-ray diffraction method. It was found that the compressive residual stress with the range between 400 and 600㎒ has distributed in the laser hardening zones and the tensile residual stress between 100 and 200㎒ has distributed in the boundary of hardening zones.