Bovine Cu, Zn-SOD was modified by 2,4-bis (O-methoxypolyethylene glycol)-6-Chloro-S-triazine (activated PEGm₂) in different rates between amino groups of SOD and PEGm₂. The purified PEGm₂-SOD that remained 80％ residual activity with 30％ modified amino groups was obtained. The fluorescence spectrum and circular dichroism of modified SOD were changed; the results of denaturation by GuHCl and of hydrolysis by pepsin showed that PEGm₂-SOD had higher stability than SOD; the results of immunology showed that the immunogenicity and antigenicity of PEGm₂-SOD decreased greatly.