Damage of fine grained hard metals WC-10%Co was investigated using the laser thermal shock method. Cemented carbide insert was irradiated with a high power laser and scanning electron microscopy was used to characterize the damage in the surface. Stress was analysis with thermo-elastic theory after thermal shock cycles. Crack initiation of cemented carbide was primarily induced by high thermal shock cycles under high rotation speed and low feed rate. Tangential stress played a primary role in crack initiation and it was demonstrated that why the thermal cracks always perpendicular to the cutting edge.