Abstract: After being sprayed with 10 mmol/L CaCl2, peanut seedlings were grown in the incubator at 42~C. Some physiological indexes were investigated every three hours, while the ultrastructure of the young leaves was observed. Results showed that Ca2+ could effectively inhibit chlorophyll content, decrease photosynthesis rate and improve or protect the activity of Ca2+-ATPase. U/trastructural observation showed that Ca2+ could stay the membrane structure of organelle such as chloroplast etc. to be destroyed effectively under the stress of high temperature. Caz+ could improve the heat tolerance of peanut seedlings.