Abstract: 5-year-old apple trees（Yanfu 3/M26/Malus hupehensis Rehd.）were treated with three different plant growth regulators to study the effect on fruit setting，which were α-Naphthalene acetic acid（NAA，10 mg ? L-1），Ethrel（600 mg ? L-1）and 6-Benzylaminopurine（6-BA，100 mg ? L-1）respectively，and all the trees were labeled with 13C and 15N to determine the characteristics of carbon and nitrogen utilization. The results showed that fruit-set rate was obviously lower than control when sprayed with 10 mg ? L-1 NAA and 600 mg ? L-1 Ethrel 14 days after bloom，while no significant difference was found on spraying 100 mg ? L-1 6-BA treatment. The 13C and 15N distribution to fruits of NAA and Ethrel treatments were 1.11%，1.22% and 0.39%，0.33% respectively，which were lower than control（9.12%，7.29%），while the 13C allocation to roots and branches were higher than control significantly. Similar regulation canbe seen on Ndff of NAA and Ethrel treatments，the treatments improved the leaves，roots and branches duty ability of 15N，and reduced the fruits duty ability of 15N. The 15N utilization rate was increased by 2.35% and 2.37% respectively when sprayed NAA and Ethrel compared with control，and the treatments promoted the vegetative growth and the duty ability of N photosynthate of the roots，leaves and branches，while no significant difference was found on the 15N utilization of 6-BA treatment.

Key words: apple, NAA, ethylene, 6-BA, 13C, 15N, absorption, distribution, utilization