Abstract: The differential expression of the caffeine synthase gene in tea plant under polyethylene glycol(PEG ) stress was analyzed by fluorescence quantitative polymerase chain reaction. The results indicated that comparing with control, the caffeine synthase gene expression increased significantly in the 12 hours and reached the maximum level 6 hours after PEG stress, and then decreased. The caffeine synthase gene transcription level in tea leaves follows a high-low pattern under PEG stress.The relative expression ratios ranged from 0.13 to 10.97.