The crop load of Japanese plums must be adjusted to ensure that the size of the harvested produce agrees with market requirements. The present study investigated the effects of gibberellic acid (GA(3)) treatment at different concentrations and during different cultivation periods after full bloom on the next year's fruit thinning and the current year's fruit quality. GA(3) was applied to plum trees at concentrations of 50 and 75 mg/L at 12 and 14 weeks after anthesis (WAA). GA(3) treatment at a concentration of 75 mg/L 14 WAA decreased harvest yield by 27.93%, whereas yields were similar to that of the control treatment under the remaining experimental conditions. The main impact was on fruit size: trees treated with GA(3) 14 WAA had significantly larger fruit than did trees under the other treatments. In consideration of the reduction to total yield, 50 mg/L is recommended for thinning purposes in Japanese plum. Fruit color, total soluble content concentration, and fruit firmness were increased after GA(3) application. The antioxidant activity of the untreated trees was higher than that of the GA(3) treated trees, whereas phenol content was determined to be similar. (C) 2015 Published by Elsevier B.V.