Abstract: Reactor pressure vessel (RPV) is an important safety barrier of nuclear power plant, and the change of fracture toughness of RPV steel in ductile-to-brittle transition zone is an essential consideration in the safety evaluation of nuclear power plant. International standards such as ASTM standard usually recommend the Master Curve method to study the change of fracture toughness of RPV steel in ductile-to-brittle transition zone. The fracture toughness test of SA-508 steel was completed by using single edge-notched bending (SEB) and compact tension (CT) specimens of different sizes in the temperature range from room temperature to −100 °C. The fracture toughness transformation law of SA-508 steel in ductile-to-brittle transition zone was studied based on the Master Curve method, at the same time, the prediction accuracy of ductile-to-brittle transition temperature obtained by the Master Curve method based on the fracture specimens of different sizes is compared, and its failure characteristics were studied by analyzing the micromorphology of the fracture specimens. It is shown that the specimen configuration and geometry have significant influence on ductile-to-brittle transition behavior of RPV steel. The Master Curve method has good accuracy in predicting ductile-to-brittle transition temperature of standard thickness specimens, but the predicted ductile-to-brittle transition temperature of small size specimens is much different from the actual ductile-to-brittle transition zone. With the decrease of temperature, the crack initiation point of both large and small size specimens comes closer to the crack tip and has a nonlinear relationship with the fracture toughness of the specimens.