We studied a assay method on the measurement of superoxide dismutase (SOD Superoxide : superoxide oxidoreductase, EC. 1. 15. 1. 1) activity with photoreduced flavin and nitroblue tetrazolium (NBT) as superoxide (${O_2}^{-}$) source and detector, respectively. The $Delta$E (1000 ng SOD$.$$min.)^{-1}$ of photoreduced flavin-NBT system was 0.08, whereas that of xanthine-xanthine-cytochrome system used broadly in experiments was 0.014. Therefore, the new method was regarded more simple and utilizable than xanthine-xanthine cytochrome system method. In the present paper, we also carried out to investigate the SOD activity and isozyme pattern for the parpose of study of leaf-burning disease in ginseng (Panax ginseng C.A. Meyer) leaves.