The influence of thermal fatigue test temperature on crack propagation behaviour 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 L-m is proposed as the index representing the susceptibility to crack initiation and propagation. The thermal fatigue tests were conducted at the maximum temperature of 600degreesC, 700degreesC and 720degreesC using as-heat treated or nitrided specimens. The ion nitrided specimen showed lower L-m 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.