Abstract: Top buds of one-year-aged seedlings of Chinese buckeye (Aesculus chinensis Bunge) and red buckeye (A. pavia L.)were placed in different cold stress conditions, and the ultrastructural changes of these top bud cells were compared under the transmission electron microscope. The results showed that the lethal temperature and duration time of top bud in Chinese buckeye was-18℃ within 72h, -30℃ within 24～72h, and-70℃ within 24 h. The ultra structure of top buds in Chinese buckeye at the lethal condition was shown as fol lows: most of cells were degraded and vacuolated. The cytoplasm was disintegrated. Although a few of cells were not disintegrated, their nuclear membrane could not be seen clearly. The lethal temperature and duration time of top buds in red buckeye was--18C within 168h.-30℃ within 168h, and-70℃ within 24 h. The ultra-structure of top buds in red buckeye at the lethal condition was shown as follows: The plasmolization was found in cells. Cell structure was blurred. The cytoplasm was disintegrated. Cell nucleus was condensed and many black particles were found in plasma membrane. It could be found from the comparison of ultra-structure in two species that the cold-resistance of Red buckeye was better than that of Chinese buckeye.