Recombinant human Cu/Zn SOD(rhCu/Zn SOD) obtained from E.coli was covalently linked with an amphipathic molecule poly-(styrene-co-maleic acid) butyl ester (SMA) via amide linkage. When 42% free amino groups of the enzyme were modified. 88% remained enzyme activity was obtained.The results of circular dichroism of rhCu/Zn SOD and SMA-rhCu/Zn SOD indicated that the structure of the modified rhCu/Zn SOD was scarcely changed. Its biological half-life in blood was prolonged 22 times.and its abilities to resist pepsin and trypsin were increased significantly.