The influence of thermal fatigue test temperature on crack propagation behavior of the surface treated tool steel for die-casting was investigated. For this purpose, thermal fatigue system consisted of induction heating and water spray quenching unit was constructed to simulate the service condition and Lm is proposed as the index representing the susceptibility to crack initiation and propagation. The thermal fatigue tests were conducted at the maximum temperature of 600°C, 700°C and 720°C using as-heat treated or nitrided specimens. The ion nitrided specimen showed lower Lm value than as-heat treated at all test temperature. But in the case of maximum and average crack length, the ion nitrided specimen exhibited higher value than those of as-heat treated specimen.