Abstract: The grind-hardening of steel 40Cr had been carried out on a conventional surface grinder with alumina grinding wheel.The microstructure of hardened layer and formation mechanism were investigated.The microstructure of hardened layer consist of the completely hardened region and the transition region.The completely hardening region is composed of fine acicular martensites,residual austenites and a few dot-like carbonides.From surface to bottom in the hardened layer,the microstructure varies from fine to coarse and then backed to fine.The mechanism is that thermo-mechanical coupling affects austenite granularity during grinding hardening and thus directly affects the microstructure of martensites.