Abstract: Abstract: Planting density and nitrogen (N) fertilizer investment are 2 important practices for cotton production, and suitable planting density and N fertilizer application rate can improve both cotton yield and N use efficiency (NUE). Using transgenic Bt+CpTI cotton cultivar CCRI 79, a field experiment with split block design was conducted in the farm of Institute of Cotton Research of Chinese Academy of Agricultural Sciences in Anyang City, Henan Province from 2013 to 2014, in order to understand the interaction of planting density and N level on seed yield and NUE of cotton. And there were 3 treatments with different planting density in main plot which were 3.00, 5.25, 7.50 plant/m2 respectively, and 4 dosages of N fertilizer were set as split block treatments which were 0, 112.5, 225.0, 337.5 kg/hm2 respectively. The results showed that LAI (leaf area index) of cotton at budding stage, flowering stage and boll-setting stage under the same planting density increased with the increase of N application rate respectively, while LAI of cotton at budding stage and flowering stage under the same N application rate significantly increased with the increase of planting density respectively. N uptake of cotton under the treatments with medium (5.25 plant/m2) and high planting density (7.50 plant/m2) in 2013 and 2014 significantly increased with the increase of N application rate, and that under the treatment with high planting density and high N application rate (337.5 kg/hm2) was the most among all treatments. When N application rate was the same, N uptake of cotton increased with the increase of planting density. Internal NUE of cotton in 2013 and 2014 decreased with the increase of N application rate. Agronomic NUE of cotton under the treatments with low (3.00 plant/m2) and high planting density significantly decreased with the increase of N application rate. When N application rate was 225.0 kg/hm2, agronomic NUE of cotton under the treatment with medium planting density was higher than those under the treatments with low and high planting density. When planting density was the same, N partial productivity of cotton significantly decreased with the increase of N application rate. Under the condition of medium (225.0 kg/hm2) and high N application rate, N partial productivity of cotton under the medium planting density was better than other 2 planting densities. Under the condition of the same planting density, N recovery efficiency of cotton decreased with the increase of N application rate. Under the condition of high planting density, physiological NUE of cotton decreased significantly with the increase of N application rate. Physiological NUE of cotton under the treatment with medium N application rate and medium planting density was higher than other treatments, while that under treatments with high N application rate increased with increase of planting density. Agronomic NUE, N partial productivity and physiological NUE of cotton under the treatment with high planting density and low N application rate were much higher than other treatments respectively, which indicated that increasing planting density and decreasing N application rate appropriately at the same time could improve agronomic NUE, N partial productivity and physiological NUE of cotton. Under the condition of low planting density, seed cotton yield increased with the increase of N application rate. Under the condition of medium planting density, seed cotton yield under medium N application rate was higher than that under high N treatment. Under the condition of high planting density, seed cotton yield decreased with the increase of N application rate. Biomass of cotton increased with the increase of planting density and N application rate, and harvest index decreased with the increase of planting density and N fertilizer application rate. Boll number per unit area under the treatment with 5.25 plant/m2 and N application rate of 225.0 kg/hm2 was much more, and seed cotton yield, lint cotton yield and N fertilizer recovery rate were superior to other treatments.