Abstract: Using the laser surface heat treatment of greycast iron, we investigate the diffusion behavior of carbon. Results show that the diffusion of carbon in the strip shaped graphite phase of matrix result in different transformation among the ambient structures, which leads to a zigzag border between the transaction/fusion zone. At the same time,when the blacker on the surface of workpiece is irradiated by the laser beam, carbon diffusion will occur from the blacker to the matrix, which effects hardness of the surface relatively low.