Abstract: The effect of thickness of compact tension (CT) specimens on creep crack growth (CCG) rate and its mechanism for Cr-Mo-V steel at 566 ℃ was investigated. The results show that the effect of specimen thickness on CCG rate is related to load level (C*). At the lower load level region, with increasing the specimen thickness, the out-of-plane constraint increased and the specimens tended to fracture in intergranular creep brittle fracture mode, and the CCG rate increased. At the higher load level region, all specimens fractured in mixed intergranular and transgranular creep ductile fracture mode, and the CCG rates basically were not affected by the specimen thickness. The CCG rate versus load level curves on log-log scale were composed of two straight lines with different slopes instead of a single linear relation. Because the straight line slope at lower load level region was smaller than that at higher load level region, it would be non-conservative if the CCG rate at lower load level was extrapolated from that at higher load level.