Abstract: Leaf color is usually used in the diagnosis of crop N status and leaf color threshold (LCT) is the basis for precise fertilizer-N application. The LCT is the optimum leaf color at which maximum grain yield is achieved. It has been reported that LCT is affected by the population size and the light intensity during grain-filling stage. The objective of this study was to determine the quantitative relationship between LCT and population size and light intensity during grain-filling. Four field experiments were conducted at Guangzhou during 2004-2005 early and late seasons. Yueza 122, a two-line hybrid rice, was arranged in eight fertilizer-N treatments. Leaf color (SPAD value), leaf area index (LAI) at heading and grain yield were measured. The results showed: (1) The LCT was different among the 4 seasons, varying between 39~45. The LCT can be derived conveniently and precisely from the quadratic curve with leaf color and grain yield being the independent and dependent variable, respectively. (2) The relationship between grain yield and population index (the product of SPAD and LAI) at heading was a quadratic curve. About 86% of variation in grain yield could be explained by leaf color, LAI at heading and sunshine hours during grain-filling. The optimum population index increased as sunshine hours increased. (3) There existed a quantitative relationship between LCT and sunshine hours and LAI. The LCT increased as sunshine hours increased, and decreased as LAI increased. The LCT for different locations and seasons might be estimated if sunshine hours and LAI were available.