We investigate the effects of laser surface remelting on the resistance to thermal fatigue of a hot working die steel H21 in this work. Laser treatment was engineered to process samples with several treated morphologies. A dominantly martensitic microstructure, nano-sized carbide and high dislocation density in the treated area, were resulted. The average microhardness of the treated area is similar to 780 HV. The thermal fatigue resistance of laser-treated samples is notably higher than their as-received counterpart. As the thermal fatigue cycle increases, the microhardness of treated area is reduced and then flatted. Crack initiation and propagation both are hampered by it. (C) 2013 Elsevier B. V. All rights reserved.