The hardened layer of the high-C (1%) steel with a thickness of 25㎛ was obtained by the irradiation of CO₂laser with a power density of 2×10^5 W/㎠ for about 0.23 second (spot size of 0.23㎜, scanning speed of 6㎝/min). When the thickness of hardened layer was compared with the temperature calculated by the conduction of heat in solid, the hardening mechanism was due to the solid-state transformation in steel.