Abstract: Effects of calcium on the heat tolerance and active oxygen metabolism of tobacco seedling leaves were studied under heat stress. The results indicated that the cell membrane was damaged in leaves and the content of MDA and active oxygen increased, but the activity of SOD and the content of chlorophyll reduced by heat stress. After seedlings were treated with 10 mmol/L CaCl2 solution, the permeability of cell membrane, membrane_lipid peroxidation and O-·2 production rate in the tobacco leaves reduced effectively. The activity of antioxidant enzyme such as SOD, POD and CAT in the leaves were still higher than that of the control seedlings.The results suggested that the increased stability of membranes and pr otective enzyme activities of tobacco leaves by Ca2+ would favour to protect cell membrane structure. EGTA, the chelating agent of Ca2+, could partly decrease heat tolerance of tobacco leaves.