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Fracture toughness is an important index to characterize the brittle state of reactor pressure vessel (RPV) steel. During the irradiation embrittlement study of RPV steel, small size compact tension (CT) specimens are generally used, because of the limited space for irradiation test. In order to understand the effect of CT specimen size on fracture toughness test result of domestic RPV steel, different sizes CT specimens of domestic A508-3 steel were tested in this paper. Size effect on fracture toughness data was studied by Beremin model method. Then the fracture toughness data normalization model (TSM, a modified toughness scaling model) of different sizes CT specimens was established. The results show that the fracture toughness increases with the decrease of sample size at the same test temperature. The normalized data of different specimens obtained by standard method has deviation. The TSM established in this paper can reduce the deviation of converted data. Key words: A508-3 steel, fracture toughness, size effect, Beremin model, data normalization model