Abstract: Three transversal cracks were found on the tread of subway wheel when the metro vehicle had run about 70 000 kilometers.For clarifying the characteristic and formation cause of the transversal cracks,the microscopic experimental analysis on the wheel tread cracks was carried out by means of the hot-acid leaching device,metallurgical microscope and scanning electron microscope(SEM).The results show that there is exceptional breaking heat on the metro wheel tread which easily leads to a heat affected zone on the tread and tensile stress in the surface layer with a certain depth.This reason will cause the thermal fatigue cracks.