Nitrogen(N) fertilizer experiments were conducted to investigate the optimal subtending leaf N concentration for fiber strength,and its relationship with activities of key enzymes(sucrose synthase and β-1,3-glucanase) and contents of key constituents(sucrose and β-1,3-glucan) involved in fiber strength development in the lower,middle and upper fruiting branches of two cotton cultivars(Kemian 1 and NuCOTN 33B).For each sampling day,we simulated changes in fiber strength,activity of sucrose synthase and β-1,3-glucanase and levels of sucrose and β-1,3-glucan in response to leaf N concentration using quadratic eqs.;the optimal subtending leaf N concentrations were deduced from the eqs.For the same fruiting branch,changes in the optimal leaf N concentration based on fiber development(DPA) could be simulated by power functions.From these functions,the average optimal subtending leaf N concentrations during fiber development for the cultivar,Kemian 1,were 2.84% in the lower fruiting branches,3.15% in the middle fruiting branches and 3.04% in the upper fruiting branches.For the cultivar,NuCOTN 33B,the optimum concentrations were 3.04,3.28 and 3.18% in the lower,middle and upper fruiting branches,respectively.This quantification may be used as a monitoring index for evaluating fiber strength and its related key enzymes and constituents during fiber formation at the lower,middle and upper fruiting branches. Nitrogen (N) fertilizer experiments were conducted to investigate the optimal subtending leaf N concentration for fiber strength, and its relationship with activities of key enzymes (sucrose synthase and β-1,3-glucanase) and contents of key constituents (sucrose and β- 1,3-glucan) involved in fiber strength development in the lower, middle and upper fruiting branches of two cotton cultivars (Kemian 1 and NuCOTN 33B) .For each sampling day, we simulated changes in fiber strength, activity of sucrose synthase and β -1,3-glucanase and levels of sucrose and β-1,3-glucan in response to leaf N concentration using quadratic eqs.; The optimal subtending leaf N concentrations were deduced from the eqs.For the same fruiting branch, changes in the optimal leaf N concentration based on fiber development (DPA) could be simulated by power functions. These functions, the average optimal subtending leaf N concentrations during fiber development for the cultivar, Kemian 1, were 2.84% in the lower fruiting branc hes, 3.15% in the middle fruiting branches and 3.04% in the upper fruiting branches. For the cultivar, NuCOTN 33B, the optimum concentrations were 3.04, 3.28 and 3.18% in the lower, middle and upper fruiting branches, respectively.This quantification may be used as a monitoring index for evaluating fiber strength and its related key enzymes and constituents during fiber formation at the lower, middle and upper fruiting branches.