According to JEAC4206-2016, in the structural integrity assessment of a reactor pressure vessel (RPV), the fracture toughness (KJc) should be higher than the stress intensity factor at the crack tip of a postulated underclad crack (UCC) near the inner surface of the RPV during a pressurized thermal shock event. Previous analytical studies show that the plastic constraint for UCC is lower than that for surface crack. Consequently, the apparent KJc for UCC is expected to be higher than that for surface crack. In this study, we performed three-point bending fracture toughness tests and finite element analyses (FEAs) for RPV steel containing a UCC or a surface crack to quantitatively investigate the effect of cladding on the plastic constraint and subsequent KJc evaluation. From the tests, we found that the apparent KJc for the UCC was considerably higher than that for the surface crack. Such a high KJc could be explained by the lower plastic constraint parameters, such as T-stress and Q-parameter, of the UCC compared with those for the surface crack. Additionally, local approach analysis showed that the KJc for the UCC was significantly higher than the master curve estimated from the fracture toughness tests using compact tension specimens.