Several nitrogen (N) field experiments were carried out in Nanjing and Anyang to study the dynamic characteristics of biomass accumulation and nitrogen uptake, and to define the dilution curve for critical N concentration (i.e. the minimum N concentration required for the maximum growth) in the reproductive organ of cotton over the period of growth. The results show that the total biomass and N accumulations were affected significantly by the application rates of N fertilizers throughout the growth period, exhibiting a sigmoid curve over time. The beginning time of fast N accumulation was 1-5 days earlier than that of biomass. The cotton lint yield was correlative with N concentrations in the reproductive organ and fluctuated with the varying N concentrations, indicating the existence of luxurious N consumption for the reproductive organ of cotton. The N concentrations increased with the increasing N application rates, and decreased gradually during the growing period. The relationship between the biomass and N concentrations can be described by a power equation. The patterns of N concentration dilution model were consistent at both experimental sites, but the model parameter values, particularly the values of a differed. The results presented in this paper indicate that a critical N concentration dilution curve for cotton reproductive organ is independent of ecological region and can be described with a power equation. The dilution curve can be used as a reliable tool for diagnosing the N status of cotton for optimal yield and quality.