Abstract: 【Objective】 There is controversy about suitable nitrogen(N) application period in cotton cultivation. Some scholars abroad recommended that the best periods of N application for cotton were the emergence and budding stage, while other researchers concluded that N application before the sowing andinitial flowering stage each half was better for cotton. N isotope tracer technique can distinguish N of crop absorption and utilization deprived from fertilizer N or soil N, and can thoroughly investigate direction of N fertilizer and distribution in cotton plant. This work was to study effects of N application ratios of the base fertilizer and topdressing and topdressing period on N absorption and yield of cotton based on N isotope tracer technique in order to provide theoretical basis for N management of cotton in North China Plain. 【Methods】 A pot trial using 15N trace technique was conducted with transgenic Bt+CpTI cotton(Gossypium hirsutum L.) cultivars, China Cotton Research Institute(CCRI 79 and CCRI 60) at the farm of Institute of Cotton Research of Chinese Academy of Agricultural Sciences in Anyang Henan in 2013.There were 4 treatments in the trial with total N application rate of 3.5 g each pot, and N application ratios of base fertilizer and topdressing at the initial flowering stage were 1:1(N1),1:2(N2) and 0:1(N3) respectively and as well as N application ratio of base fertilizer and topdressing at the budding stage 0:1(N4). Effects of different N application strategy on 15N absorption, 15N recovery efficiency, biomass accumulation of cotton at the initial flowering stage as well as the harvest stage and seed cotton yield at the harvest stage were examined. 【Results】 Percentages of N derived from fertilizer(Ndff) of different organs of cotton plants at the initial flowering stage are significantly increased with the increase of fertilizer rate of the basal N, and amplitude of the percentages of Ndff is 25.88-42.45. Percentages of Ndff of cotton plants at the harvest stage are significantly increased with the increase of fertilizer rate of the topdressing N, and amplitude of the percentages of Ndff of cotton plants is 26.92-54.14. Percentages of Ndff of cotton plants at the harvest stage under N3 and N4 are significantly higher than those under N1 and N2. Seed cotton yields of cotton plants of the 2 cultivars with N2 are higher than those of the other 3 treatments, and there is not significant difference between those of N2 and N1. Biomass accumulation amounts of cotton plants of the 2 cultivars with N2 are not significantly different from those of N1 and N3 respectively. 15N uptake amounts of cotton plants of the 2 cotton cultivars with N3 and N4 at the harvest stage are significantly higher than those with N1 and N2 respectively. 15N recovery rates of cotton plants of the 2 cotton cultivars with N3 and N4 at the harvest stage are significantly higher than those with N1, while 15N recovery rates of cotton plants of the 2 cotton cultivars with N2 at the harvest stage are higher than those with N1, and there is not significant difference between N2 and N1. 15N recovery rate of soil after the harvest with N2 is less than that with N1, and there is not significant difference between N2 and N1. 【Conclusions】 When N application ratio of the basal fertilizer and topdressing at the initial flowering stage is 1:2 in the pot trial, the seed cotton yield and 15N recovery rate of cotton are higher than those of the conventional N application ratio 1:1. When N application ratio of the basal fertilizer and topdressing at the initial flowering stage is 0:1, the 15N recovery of cotton is the highest among the 4 treatments, but the seed cotton yield is not improved. The results above in the pot trial should be further tested in field trial