Abstract: The changes of chlorophyll and the resistance related physiological indicators in leaves of diploid and autotetraploid of watermelon during growth process were studied. The results showed that the contents of chlorophyll a，chlorophyll b and total chlorophyll on unit leaf area of tetraploid were all significantly higher than that of diploid in watermelon，while the contents of carotenoids were slightly lower. The activities of Superoxide Dismutase，Peroxidase and Polyphenol oxidase in leaves at three growing periods were all significantly higher than that of diploid in watermelon，and the activity of Catalase was slightly higher，while the content of Malondialdehyde was significantly lower. These indicated that the tetraploid is superior to the diploid in photosynthesis and resistance in watermelon．