Abstract: Trace amount of tin in nuclear power steels was determined by HG-AFS. Sample was dissolved with mixed acid of HCl-HNO3(3+1) in the presence of tartaric acid. Mixed solution of 50 g·L-1 thiourea and ascorbic acid was used as masking agent and 20 g·L-1 potassium borohydride was used as reductant. The generation of SnH4 was carried out in a medium of pH 5.0-5.5. Linear relationship between values of fluorescence intensity and mass concentration of tin was kept in the range within 50 μg·L-1, with detection limit (3s/k) of 0.4 μg·L-1. The proposed method was applied to the determination of trace amount of tin in samples of nuclear power steels and standard samples of stainless steels, the results obtained were in consistency with the certified values and also with the results found by ICP-AES, giving values of RSD′s (n=8) less than 4.5%.