Abstract: In order to study the effects of exogenous 2，4-epibrassinolide on the heat tolerance of peony，the 3-year-old Paeonia ostii ′Fengdan′ container seedlings was taken as materials which treated with 0. 1，0. 5，1. 0，1. 5 mg/L of epibrassinolide through spraying them on the leaves，under the artificial climate simulating the heat stress（40 ℃ /35 ℃，day / night），the related physiological and biochemical indexes and ultrastructure in leaves of Paeonia ostii ′Fengdan′ were determined in the experiment. The experimental results showed that: exogenous epibrassinolide increased net photosynthetic rate and reduced the relative electrical conductivity of leaves of Paeonia ostii ′Fengdan′. The accumulation of malondialdehyde was inhibited effectively and the soluble sugar content was decreased. The content of soluble protein was increased and the superoxide dismutase（SOD）activity was enhanced. The exogenous epibrassinolide could slow down the rate of Paeonia ostii ′Fengdan′ cell damage by the high temperature and let the cells remain relatively intact structure. It could also inhibit the disintegration of cell membrane，the deformation of chloroplast and mitochondrial. Appropriate concentration of exogenous epibrassinolide can enhance the heat resistance of peony， and the concentration of 1. 0 mg/L is the best.

Key words: Paeonia ostii ′Fengdan′, epibrassinolide, high temperature stress, heat resistance