Abstract: Two clones seedlings of Chinese fir (Cunninghamia lanceolata (Lamb.)Hook.)with different growing rates (Fast growing pattern CHAO22 and slow growing pattern KAI 4) were used to analyze the change of two importent photosynthate sucrose and starch contents in leaves by day and night. The results showed that:(1) The sucrose content in leaves of fast growing seedling was 80% more than that of the slow growing seedlings.But the starch content of the latter was 4～17% higher than the former one.(2) The sucrose and starch contents in leaves were obviously accumulated in the daytime.The sucrose content at noon was the most.The sucrose contents of fast growing seedlings and slow growing seedlings were 85% and 12% higher than those in the morning,respectively.But the starch content in leaves was the highest in the evening and was over 80% higher than that in the morning.(3)The starch contents in leaves and young shoot of fast and slow growing seedlings were decreased in a single night.They were decreased by 31% and 13% (fast growing seedlings) and by 43% and 24% (slow growing seedlings)respectively. There wasn′t obvious change of sucrose content in leaves of the two kinds of clones in the night,but the starch content in young shoot was obviously decreased. In addition,the relationship between the sucrose content and growth potential was discussed.