Abstract: 【Objectives】The aim for this study was to elucidate physiological mechanism on photosynthate synthesis and transport of subtending leaves of cotton boll (SLCB) under cool temperature for nitrogen fertilizer, and provide theoretical support for reasonable nitrogen application.【Methods】 To study effect of nitrogen rates on photosynthate contents and nitrogen concentrations in subtending leaves of cotton boll (SLCB) and their relationship to cotton yield and quality under different planting dates, a field experiment is conducted with two cotton cultivars, Kemian 1 and NuCOTN 33B, in the Yellow River Valley (Anyang), China. The cotton seeds were sowed on 25-Apr and 25-May in 2005 and 2007, which could result in different growth temperatures for the boll and its SLCB at the same fruiting branches, and three N levels, N 0, 240 and 480 kg/hm2, standing for low, medium and high nitrogen level (N0, N240 and N480), were applied at the flowering-boll development stage. 【Results】 1) The sucrose contents in the SLCB have a single trend following increasing nitrogen rates, and the starch contents in the SLCB are increased in the 25-Apr planting date. With the delayed planting date, the differences of the sucrose and starch contents between N240 and N480 are non-significant, but their contents are significantly higher than those of N0. In the present research, the sucrose content of the SLCB at 24-45 days after anthesis (DAA) has a significantly positive correlation with nitrogen concentration, and the correlation coefficient is decreased with the increase of DAA. In addition, the transport capacity of sucrose (Tn) has a significantly negative correlation with the nitrogen concentration at 17-24 DAA, and has a positive correlation with the nitrogen concentration at 31-52 DAA (P0.01). These results indicate that an optimal leaf nitrogen concentration is favorable for carbon accumulation and export. 2) In the 25-Apr planting date, boll number, the boll weight and lint yield are decreased in N0 and N480, and non-significant differences are observed in the two treatments. However, in the late planting date of 25-May, the boll weight and lint yield in the treatment of N480 is improved as well as fiber strength, and micronaire values is optimized, thus suggesting that nitrogen application has a compensatory effect on cool temperature due to late planting. 【Conclusions】 In this study, there is an interaction on cool temperature due to the late planting and nitrogen application for carbohydrate, cotton yield and quality. The planting date is the dominant factor, and nitrogen fertilizer has a compensatory effect on cool temperature. With the delayed planting date, the boll weight, lint yield and fiber quality under the treatment of N 240 kg/hm2 are declined, which is mainly due that sufficient non-structural carbohydrates (such as sucrose, hexose and starch) are stored in the SLCB, and carbohydrate might not be exported efficiently to cotton boll. Through increasing the nitrogen application, the decreased degree of fiber strength is less and the micronaire value is optimized, mainly due to the regulated nitrogen concentration and the improved recycling capacity of carbohydrate.