Abstract: This study was carried out to investigate the effects of fruit thinning on fruit sugar accumulation and leaf photosynthetic characteristics in pears（Pyrus pyrifolia Nakai.‘Hosui’）. Plants grown in the field were subjected to heavy-thinning（HT treatment），moderate-thinning（MT treatment）and non-thinning（control）. Results showed that single fruit mass in HT and MT treatments increased significantly by 172.5% and 95.9% respectively compared with control，and total soluble sugar content also increased by 31.8% and 20.7% respectively. The components in total soluble sugar were also found to be affected，sucrose content increased by 134.8% and 91.5% in HT and MT treatments compared with control，and sorbitol content increased by 53.2% and 26.7%，while glucose content decreased in HT treatment by 45.0% and 22.7% to other two treatments. Fruit thinning at different stages showed similar effects as above treatments，earlier thinning could result in higher content of，even 90 days after anthesis（DAA），fruit thinning can still promote the sugar accumulation. As for leaf photosynthetic metabolism，leaves on bourse shoot in control treatment could maintain the highest net photosynthetic rate（Pn）till 80 DAA，however it decreased sharply at 110 DAA. Under an electron microscope，increased osmiophilic particles of chloroplasts and fuzzy lamellar structure were observed in the leaves of control treatment at 110 DAA，which was considered to be related to the drop of photosynthetic rate and leaf senescence.

Key words: pear, fruit thinning, sink-source relation, sugar accumulation, photosynthetic characteristics, leaf, senescence