During cancer therapy, gamma-ray irradiation and treatment of anti-cancer chemicals destroy the normal cells as well as cancer cells. In this study, we investigated the effect of epigallocathechin-gallate(EGCG) extracted from green tea, which is known to have anti-cancer and anti-oxident activities, in order to find out the feasible method to protect the normal cells and to kill the cancer cells efficiently. We investigated the effect of EGCG on the leukemia cell growth and cell necrosis, especially when treated along with gamma radiation. The EGCG inhibited the leukemia cell, HL-60, growth at the appropriate concentration while it exhibited no influence on the normal cell growth. More significantly, it enhanced leukemia cell necrosis when its treatment was combined with gamma irradiation. Simultaneous treatment of EGCG and gamma radiation increased leukemia cell necrosis up to 35% compared with separate treatments. These results suggest that drinking of green tea or co-treatment of EGCG during gamma irradiation therapy may result in better prognosis through enhancement of the tumor cell necrosis and protection of the normal cells.