Abstract: The critical fracture stress of coarse grain heat-affected zone (CGHAZ) of 12Cr10Co3W2Mo heat resistant steel weld was investigated by implant test when the preheating temperature was room temperature, 100℃, 200℃ and 300℃, respectively. The highest microhardness in heat-affected zone was measured; the microstructure in the CGHAZ was observed; and the fracture surface of implant samples was detected with scanning electron microscopy (SEM). The results indicate that the critical fracture stress increased linearly with the increase of preheating temperature. The microstructure in CGHAZ was lath martensite and the microhardness was highest. The microhardness of CGHAZ decreased slowly as the preheating temperature increased. Under different preheating temperatures, the crack initiation zone in implant samples was mixed cleavage and quasi-cleavage hydrogen-induced cracking (HIC) morphology. And with the increase of preheating temperature, the HIC features disappeared gradually.