Abstract: In this study, young pepper (Capsicum annuum L. cv. Xiang-Yanl0) leaves were used to examine the effects of exogenous Ca2+ pretreatment on electrolyte leakage, ascorbate(AsA) and glutathione(GSH) contents, and the change in Ca2+ distribution in mesophyll cells under heat stress. The results show that exogenous Ca2+ pretreatment can alleviate the damage of cell membrane caused by heat stress. The amount of GSH and AsA in calcium-treated leaves under heat stress was higher than that in control leaves. In the control pepper seedlings growing under normal temperature, deposits of calcium antimonate, an indicator for Ca2+ localization, were observed within the vacuoles, intercellular spaces, and chloroplasts. This indicates that Ca2+ pretreatment could induce changes in Ca2+ distribution and significantly increase the Ca2+ level in intercellular spaces, vacuoles and chloroplasts. When calcium-treated leaves were under heat stress at 40˚, the amount of Ca2+ located in intercellular space decreased, and most of them transferred to cytosol and chloroplast across the plasmalemma. The results suggest that exogenous Ca2+ results in changes in Ca2+ distribution between inter-and intracellular spaces and, as a result, the ultrastructure of mesophyll cells could be protected from damage caused by heat stress.