Seedlings from the tetraploid Cymbidium hybrid and the original diploid Cymbidium hybrid were exposed to high temperature（40 ℃/30 ℃,12 h light/day).The effects of stress on plant physiological and biochemical characteristics,including the chlorophyll content,the membrane damage degree,the osmosis-regulation substance content and the activities of protective enzymes,were analyzed to assess the difference of heat resistance between the diploid and tetraploid.The results showed that high temperature stress decreased the chlorophyll content of both seedlings,while tetraploid showed less decrease.The malondialdehyde (MDA) content was increased in both diploid and tetraploid plants,but diploid increased more.The O^-·₂ release rate was decreased in both diploid and tetraploid seedlings and decreased faster in diploid.The increasing ranges of leaf proline content,soluble sugar,soluble protein,and activities of catalase (CAT) and superoxide dismutase (SOD) were all higher in tetraploid,while decreasing ranges of POD activity was notable in diploids.Our results indicated that tetraploid was more powerful than diploid in heat resistance because tetraploid showed higher competence in cell osmosis-regulation,protein synthesization and stabilization,and reactive oxygen species (ROS) elimination.