The fracture toughness in the heat-affected zone (HAZ), which is located under the weld overlay cladding on the inner surface of the reactor pressure vessel (RPV), was evaluated by considering the inhomogeneous microstructures of the HAZ. In this regard, 0.4T-C(T) specimens and Mini-C(T) specimens were manufactured to clarify the effect of the HAZ on fracture toughness. The fracture toughness was evaluated using 0.4T-C(T) specimens targeted on microstructural distribution due to the cladding bead location. The reference temperature (To) of HAZ at middle region of cladding bead was 12°C higher than that at overlap cladding beads. The fracture toughness was evaluated using Mini-C(T) specimens focused on the microstructural difference with distance from the boundary between the cladding and the base metal. The To value of the central position located at 3 mm to 7 mm from the boundary between the cladding and the base metal was higher than those of the other positions in HAZ. However, the To values of HAZ for both 0.4T-C(T) and Mini-C(T) specimens were significantly lower than that of the base metal at a quarter thickness by 40°C-60°C. Compared with the literature data that indicated fracture toughness at the inner surface without overlay cladding and the base metal at a quarter thickness, this study concluded that the HAZ under the overlay cladding showed no detrimental effect on the conservatism of the structural integrity assessment of RPV steel.