Cotton (Gossypium hirsutum L.) fiber plays an important role in the textile industry. Cotton fiber comes from the individual cotton bolls, so boll weight is a factor which can affect fiber yield. Boll weight is affected by genotype, environmental conditions, management practices, and other factors. Today, with superior cultivars and stable climatic and environmental conditions, management practices have become important in improving fiber yield. Nitrogen, the most important crop nutrient, has a regulatory role in crop growth. Crop yield is very commonly controlled by regulating the field application of nitrogen. The use of too much or too little nitrogen can decrease crop growth and yield. Thus, it is very important to find the optimal nitrogen levels needed to maximize fiber yield. Boll weights are controlled by both source and sink strength. Soluble sugar content, sucrose content and sucrose phosphate synthase activity are important indicators of source strength in the leaves subtending cotton bolls. Also, sucrose content and sucrose synthase activity are important indicators of sink strength in cotton fibers. Higher source and sink strength are beneficial in improving fiber yield, so finding the optimal nitrogen level is necessary for each indicator of source and sink strength. The nitrogen status of a cotton plant is more accurately reflected by the nitrogen concentration in the leaves subtending the cotton bolls than by the nitrogen application rate. So we tried to establish a quantitative relationship between leaf nitrogen concentration and each indicator to determine the optimal nitrogen level needed to achieve maximum boll weight. A field experiment was conducted to study the relationship between leaf nitrogen concentration and source and sink strength during boll development. Three cotton cultivars (Dexiamian 1, Kemian 1 and NuCOTN 33B) and five N application rates (0, 120, 240, 360 and 480 kgN/hm²) were used. The result showed that as leaf nitrogen concentration increased, the indicators of source and sink strength initially increased and then decreased when measured at 10, 17, 24, 31 and 38 DPA (days post anthesis). As an exception, fiber sucrose content decreased initially and then increased at 45 and 52 DPA. The relationships between each indicator and leaf nitrogen concentration followed quadratic curves (Y=ax²+bx+c), in which Y stands for the content or the activity of source and sink strength indicators, x is the leaf nitrogen concentration, and a, b and c are parameters determined by the cotton cultivar. The results show there is a leaf nitrogen concentration for each indicator of source and sink strength that is the optimal theoretical value. The optimal leaf nitrogen concentration for each indicator can be determined by using the equations. The differences between the optimal leaf nitrogen concentration for each indicator were small before 45 DPA (38 DPA for Dexiamian 1). The equations for the optimal leaf nitrogen concentration in relationship to time (DPA) for Dexiamian 1 (before 38 DPA), Kemian 1, and NuCOTN33B (before 45 DPA) were N_Dexiamian1=7.2263DPA^-0.276(R²=0.9805^{* *}), N_Kemian1=7.23DPA^-0.3026(R²=0.9861^{* *}), and N_NuCOTN33B=7.0997DPA^-0.2814(R²=0.9807^{* *}), respectively. However, in the remainder of the boll development period, the optimal leaf nitrogen concentration for source strength in the subtending leaf of bolls was quite different when compared with the optimal leaf nitrogen concentration for sucrose synthase in the fiber. This research suggests that we could optimize leaf source strength and fiber sink strength to improve boll weight by regulating nitrogen concentration of the subtending the leaf boll.