Abstract: The nuclear reaction pressure vessel has induced radioactivity after service, and small size specimens are required to be used for safety and improvement of material utilization. However, due to the submillimeter size, size effects will inevitably occur. It will result in differences in mechanical property test results in comparison to the standard specimens, and hinder the practical applications. Thus, this paper aims to reveal the tensile size effect mechanism of small size specimens of A508Ⅲ steel which are used for nuclear reaction pressure vessels, and to establish the relationship between sample size and mechanical properties. The thickness of the small size specimens was changed based on Small SpecimenJapan (SSJ), and the different grain sizes were obtained by changing the heat treatment process. In order to avoid the influence of the surface state on the test results, the small size specimens were processed by electrical discharge machining with a slow cutting speed, and then polished. The mechanical properties of small size specimens with different thicknesses and grain size values were tested at room temperature. Before the tensile test, a mixture of alumina and alcohol was sprayed on the surface of the small size specimens to create speckles and facilitate the extensometer to identify the deformation of the gauge length of the small size specimens. The results show that, as the austenitizing temperature increases from 900 ℃ to 1 000 ℃, the grain size of A508Ⅲ steel increases from 18.12 μm to 33.21 μm. The mechanical property of small size specimens with different heat treatment temperatures and different thicknesses exhibits an obvious size effect. As the grain size is refined, the grain size effect is produced and the internal grain boundary model can better explain its mechanism. As the thickness of the small size specimen decreases, the feature size effect is produced and the surface layer model can better explain its mechanism. In addition, a key parameter λ comprehensively described the grain and feature size effects of small size specimen was introduced to establish the mechanical constitutive model which is based on the swift relation. Taking the ASTM standard sample as the reference object, the normalization models of yield and tensile strength of the small size specimen between the standard specimen were obtained, which could expect to provide a reference for the conversion of mechanical property. The calculated results of the normalized model can be well matched with the experimental data, which proves that the normalized model is representative.