The experiments were conducted to explore the effects of three different photoperiods(16h,long day;12 h,middle day;8 h,short day) on superoxide dismutase(SOD) and peroxidase(POD) activities in fall dormancy alfalfa(Medicago sativa L.cv.Vernal).The results showed that: SOD and POD activities of Medicago sativa L.cv.Vernal were highest under 8h day-length,medium under 12h day-length,and lowest under 16h day-length;under the same day-length,SOD and POD activities of Vernal alfalfa increased with the elongation of growth period.The findings indicated that a significant relationship between SOD and POD activities and photoperiod for alfalfa,short day photoperiod can improve the activities of SOD and POD.