Abstract: To illuminate the injury mechanism of Rhododendron Subgenus Hymenanthes under high temperature stress,4-year-old seedlings of R.simiarum Hance and R.jinggangshanicum Tam were planted in artificial climate chambers and their physiological and biochemical changes were determined under different high temperature stress.The results showed that:(1)The contents of malondialdehyde(MDA),hydrogen peroxide(H₂O₂),proline(Pro)and soluble protein in leaves of both species increased when the temperature increased.The contents and changes of H₂O₂ and soluble protein in R.simiarum leaves increased less than those in R.jinggangshanicum,whereas the changes of MDA and Pro in both species were independent of temperature.(2)The contents of ascorbic acid(AsA)in leaves of the two species were negatively correlated with temperature,with R.simiarum decreased less than R.jinggangshanicum.(3)The activities of superoxide dismutase(SOD),peroxidase(POD)and ascorbate peroxidase(APX) of R.simiarum leaves increased more than those in R.jinggangshanicum leaves.However,the opposite trend occurred for the activity of catalase(CAT),with R.simiarum increasing less than R.jinggangshanicum.These results indicated that the membrane lipid peroxidation in leaves of both species increased when the temperature increased,but the active oxygen quenching ability of the enzymatic and non-enzymatic system in R.simiarum was better than that in R.jinggangshanicum.