Abstract: The susceptibility of Cr Ni-Mo-V steels to stress relief cracking was studied by the stress-relaxation test.The fracture surface was examined with a scanning electron microscope and the composition of grain boundayr face was analyzed by the energy dis-persion means.From the test data,the authors found the current conception that secondary precipitation hardening weakened the grain boundary and that reheat cracking was the result of embrittlement caused by tempering might be incorrect.Stress-relief cracking is mainly due to the lowering of grain boundary high temperature strength and to the embrittlement of grain boundary through overheating in welding.