Abstract: Early cracking failure occurred to the 8407 steel mould for model cars, so that rough selvedge generated on the outer edge of lamp cup positioning of model cars, seriously affecting the appearance quality of the model cars. In order to find out cracking reasons of the mould, the structure and the technological characteristics of the mould were inspected and analyzed by macroscopic examination, chemical composition analysis, hardness testing, energy spectrum analysis, metallographic examination and so on. The results show that:the cracking reasons of the mould were that the pulse current for electrical processing was too large and a serious quenching layer generated on the inner wall of the hole, resulting in the tensile stress cracking; the matrix structure was bulky because of the overheating, which reduced the strength and toughness of the material and further shorten service life of the mould.