The experiments including soil application N, thinning and priming leaf treatment, using eight -year-old apple trees (M. domestica Borkh.cv.Red Fuji/ M. hupenensis Rhed) as materials were carried out to study the effect of N on fruit development. The main results were as follows: on heavily thinned trees, SS activity was independent of N and priming leaves treatments. The results show that the carbohydrate restriction of source to sink activity did not exist, and N application did not stimulate fruit growth rates relative to those on nonfertilized trees; however, N fertilization resulted in a longer fruit development period and increased the growth potential of individual fruit by 20.8% (fresh weight) and 14.1% (dry weight) vs. controls ; in unthinned trees, SS activity was increased by N fertilization but decreased by priming leaves treatment , so the carbohydrate restriction of source to sink activity existed, N fertilization increased the average single fruit weight both by extending the fruit develo The experiments including soil application N, thinning and priming leaf treatment, using eight -year-old apple trees (M. domestica Borkh.cv.Red Fuji / M. hupenensis Rhed) as materials were carried out to study the effect of N on fruit The main results were as follows: on heavily thinned trees, SS activity was independent of N and priming leaves treatments. The results show that the carbohydrate restriction of source to sink activity did not exist, and N application did not stimulate fruit growth rates relative to those on nonfertilized trees; however, N fertilization resulted in a longer fruit development period and increased the growth potential of individual fruit by 20.8% (fresh weight) and 14.1% (dry weight) vs. controls; in unthinned trees, SS activity was increased by N fertilization but decreased by priming leaves treatment, so the carbohydrate restriction of source to sink activity existed, N fertilization increased the average single fruit weight both by extendin g the fruit develo