Abstract��Fracture surface was analyzed for the cracks which were produced during the flame cutting, and several comparing experiments between the failure and undamaged cutting surface were done to find the reason causing to the failure of cutting. As a result, there was big difference between the failure and undamaged flame cutting surface, such as metallic structures and micro-hardness in surface zone, and furthermore, white eutectic phase was observed in the failure flame cutting surface. In the sum, it was concluded that the cutting temperature which was more than steel plate melting was the reason why the flame cutting fails.