Abstract: In the process of machining, a crack was found on the surface of TA15 titanium alloy forging. Based on the analysis of macro and micro features of crack and fracture, metallographic microstructure examination and hardness testing, the mode and cause of the crack were determined. The temperature range of the crack initiation stage was obtained through the analysis and contrast of fracture temperature color and micro area composition. The results show that the crack of TA15 titanium alloy is a folding crack forming during forging, and it propagated during forging. The crack was mainly caused by the involvement of the oxide skin which hadn't been cleaned up from the surface. The formation temperature of brownish red fracture on the left side of the crack is higher than 800℃, and that of the gray fracture on the right is not higher than 500℃.