The nuclcase I ( EC 3.1.30.2 ) and RNase I ( EC 3.1.27.1 ) from Black locust (Robinia pseudoacacia L. ) seedling were purified 11.8 and 10.6 folds by ammonium sulfate fractionation, Sephadex G-25, Scphadex G-100 and CM-ccllulose column chromatography. Photographs of SDS-polyacrylamide disc gels stained with Coomassic blue R-250 after elcctrophoresis of the nucleases I and RNase I preparation showed single and two bands, respectively. The molecular weight of the nuclease I was 44000. The enzyme gave pH optima of 6.2 and temperature optima of 70℃ . The nuclcase I was at least composed of 17 amino acids. Form the amino acid compositions, it was found that the enzyme molecular contains the largest amounts of Aspartic acid( residue 42), least amounts of cysteine and methioninc( residue 3.2 and 6.2). Partly purified RNase I gave pH optima of 5.0 and temperature optima of 50℃. Sulfpydryl compounds can promoted acitivitics of the two enzymes. Of a number of two valent cations, Hg2+, Sn2+ and Cu2+ ( 10m mol ?F1) produced markly inhabiting effect on activity of the two enzymes, Ca2+, Ba2+ and Zn2+ ( 10m mol ?F1) resulted in slightly inhabiting effect on nuclcase I , and resulted in stimulating effect on the RNase I . Metal chelating agent EDTA lack of effect on activities of the two enzymes implies that they don’t contain any metal ions.