Abstract: In this Paper we have analysed the cracking cause of the steam turbine disc made of 34CrNig Mo steel and suggested that it may be related to stress corrosion cracking. The fracture tough-ness Kio at room temperature and Jio at both room and service temperature of the disc were determined respectively with big-size C. T. specimens and small-size three-point bending specimens. The stress corrosion cracking propagation rates do/dt were also measured. The stress intensity factor at the disc crack tip was approximately calculated by using stress superposition theorem. Conseguently the critical crack length and residual service life were evaluated.