Abstract: This study evaluated the physiological effects and biochemical characteristics of functional axial leaf and different position sympodial leaf as affected by applying different levels of nitrogen fertilizer(0 kg/ha Low N;189.50 kg/ha Middle N;395.0kg/ha High N) using Bt-transgenic hybrid cotton variety 33B.The differences of leaf area index and leaf physiological index(chlorophyll,dissoluble protein and MDA) were studied with different levels of nitrogen across the growing season were studied using a randomized block design.The leaves were divided into four classes according to their positions: stem leaves,top sympodial leaves,middle sympodial leaves,lower sympodial leaves.The main results were as follow:(1) Under the conditions of this experiment,full boll stage was the turning point of every index,indicating that this was the starting phase of leaf senescence;(2) The content of chlorophyll and dissoluble protein of the leaf decreased significantly and MDA content increased when nitrogen was deficient,which accelerated the process of senescence;(3) Compared with normal level of nitrogen,excessive nitrogen would increase senescence rate of lower sympodial leaves and aggravate the trend of senescence for the whole plant.The top sympodial leaves were most sensitive to nitrogen deficiency.Therefore,proper nitrogen applications would delay cotton leaf senescence.