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The effects of fruit thinning on fruit drop, leaf carbohydrates concentration, fruit carbohydrates concentration, leaf nutrient concentration and fruit quality in pummelo cultivar Thong Dee growing in Nakhon Pathom province, Thailand, were studied during January-August 2013. The results showed that 50% fruit thinning by hand at 1 month after fruit set increased percent of fruit retention throughout fruit development. At 2 month after fruit set, 50% fruit thinning gave 62% of fruit retention, which were significantly higher than no thinning (36.60%), whereas, 50% fruit thinning gave 40% of fruit retention at 6 month after fruit set, which significantly higher than no thinning (20.2%). A significant difference in leaf carbohydrate concentration was found in 3-6 month after fruit set. At 3 months after fruit set, 50% fruit thinning gave significantly higher leaf carbohydrate concentration (81.80 mg.g-1) than no thinning (73.56 mg.g-1), whereas, at 6 month after fruit set, 50% fruit thinning gave significantly higher leaf carbohydrate concentration (128.92 mg.g-1) than no thinning (102.90 mg.g-1). Although, 50% fruit thinning gave no effect on peel and pulp dry weight including peel and pulp carbohydrates concentration during fruit development. For plant nutrient analysis, 50% fruit thinning gave significantly higher leaf nitrogen (N), phosphorus (P) and potassium (K) concentration than no thinning during 3-6 month after fruit set. However, 50% fruit thinning had no effect on fruit weight, fruit circumstance, fruit diameter, titratable acidity (TA) and total soluble solid (TSS). It was concluded that 50% fruit thinning increased percent of fruit retention and may have an effect on the accumulation of leaf carbohydrates, leaf N, leaf P and leaf K concentrations in pummelo cultivar Thong Dee.