Abstract: The small punch test (SPT) has a broad prospect in evaluating the mechanical properties of materials before and after irradiation in the nuclear industry. Due to the limitation of ion irradiation depth, SPT specimens with a thickness of 0.1 mm can be used. Focused on the size effect of small punch test specimens for ion irradiation, six materials were tested to verify the feasibility of using a database method to determine the strength of small punch test specimens with a thickness of 0.1 mm, and the size effect and critical grain size in the test were analyzed. The conversion methods of 0.1 mm thick SPT and conventional tensile strength were established by the database method. It was found that the percentage error on estimation of yield strength by SPT is linearly related to the grain size of the sample, and the maximum percentage error on estimating tensile strength was 3.62%, the minimum and maximum errors were 4.84% and 15.9% for yield strength , respectively. The critical grain size for estimating yield strength was 7.4 μm by using SPT.