Fruit thinning in Siam mandarin (Citrus nobilis var microcarpa L.) has been applied in order to improve quality of the fruit involving bigger or higher fruit weght, the sweet of the fruit hence higher market price. The experiment of hand thinning was conducted in farmer's orchard in Bayung Gede village, district of Kintamani, Bangli regency, Indonesia province from 18 June until September 2015. The objective of the experiment was to investigate the effects of stage of fruit thinning and percentage of retained fruitlets on quantity and quality of fruits of siam mandarin cultivar. A randomized complete block design with three replicates was used in the experiment. Two treatment factors were the stage of fruit thinning (stage of fruitlet size of 10 mm, 30 mm and 50 mm) and percentage of retained fruitlet on a branch (100%, 75%, 50% and 25%). Results of experiment showed that thinning at the stage of 30 mm fruitlet size significantly increased fruit diameter fruit diameter, weight fruit-1, and fruit weight bunch-1. Thinning at the stage of 50 mm fruitlet size could only increase sugar content in the fruit of siam mandarin when retained fruitlets on the branch were 25%. Thinning at the stage of 10 mm and 30 mm fruitlet sizes resulted in higher ratio of sugar:acid content (7.34 and 7.39 respectively) than thinning at that of 50 mm. The size of fruit (diameter and fruit weight) and ratio of sugar:acid content were not significantly affected by percentage of retained fruitlets on a branch.