AbstractTransferability of fracture toughness data obtained on small scale specimens to a full-scale cracked structure involves both in-plane and out-of-plane constraint effects. Both in-plane and out-of-plane constraint effects of a crack in a reference reactor pressure vessel (RPV) subjected to pressurized thermal shock (PTS) are analyzed by two-parameter and three-parameter methods. T11 (the second term of William's extension acting parallel to the crack plane) generally displays a reversed relation to the stress intensity factor (SIF) with the transient time, which indicates that the loading (SIF) plays an important role on the in- plane constraint effect. T33 (the second term of William's extension acting along the thickness) displays a different relation to T11 during the transient. The results demonstrate that both in-plane and out-of-plane constraint effect should be analyzed separately in order to describe precisely the stress distribution ahead of the crack tip. The local approach to fracture, i.e. σ*-A* model is used to predict the in-plane and out-of-plane constraint effect by considering the micro mechanism of cleavage fracture.