Abstract: The preparation and properties of polyethylene glycol (PEG) modified superoxide dismutase (SOD) are reported. It involves the formation of a reactive intermediate by reacting PEG with succinic anhydride and p-nitrophenol, then coupling of the intermediate with SOD. The remaining activity of PEG-SOD ranged from 60% to 80%, and showed a much higher stability than natural SOD even in higher temperature and acidic or basic surroundings.