Abstract: Standard specimens are usually used to reflect the mechanical properties of materials. However, because of the size effect between standard specimens and small size specimens, neither the repeatability nor accuracy obtained from small size specimens is unreasonable to reflect the mechanical properties of materials. In order to meet the miniaturization of supervision specimens for nuclear reactor pressure vessels, it is necessary to establish a mechanical property relationship model between small size specimens and standard specimens.In this paper, the small size specimens (small specimenJapan, SSJ) with the thickness of 0.75 mm were taken as examples to construct the tensile test of smallsize specimens and conduct accurate verification. The microstructure of A508Ⅲ steel was analyzed, and the grain size of specimens was counted by metallographic photograph. The tensile mechanical behaviors of small size specimens and standard specimens of A508Ⅲ steel were analyzed. The evolution law of crystal orientation in specimens before and after tension was quantitatively characterized by electron backscatter diffraction (EBSD), and the mechanical constitutive model of A508Ⅲ steel coupled the crystal orientation effect with the size effect was established. The results show that, with the decrease of geometry size of specimens, the mechanical properties of the tensile specimens exhibit the size effect and the crystal orientation effect. As the number of grains contained in the thickness direction of SSJ decreases, the proportion of surface grains increases, which leads to a change in the contribution of surface grains to the mechanical properties of the material and presents a size effect. During the tension process, the number of grains in SSJ with the crystal orientation of 〈110〉 increases by 28%, while the number of grains with the crystal orientation of 〈100〉 and 〈111〉 decreases by 9% and 19%, respectively, which results in the change of the contribution of crystal orientation to its tensile plastic deformation and elongation and producing the crystal orientation effect. The previous studies show that the HallPetch relation model and surface layer model work effectively for the size effect. Based on the HallPetch relation model and surface layer model, a new model was proposed. The established mechanical constitutive model of A508Ⅲ steel coupled the crystal orientation with the size effect can accurately describe the tensile mechanical behavior of SSJ and has the high accuracy and reliability. The prediction error is less than 5%, which can provide a reference for the prediction of tensile mechanical properties of SSJ and the construction of normalization model. For practical applications, different models together with the size effect coefficients and the grain orientation fraction are modified to work accurately for the prediction.