Abstract: Soil incubation experiment designed with 3, 3 simplex centroid desig n method was adopted to assess the effects of combination of urea and two kinds of CRFs (polymer-coated urea, D90 and D60) on the contents of NH4+-N, NO3--N and mineral N(Nmin) in soil. The initial solubility of seven CRFs used in t his ex periment was less than 12.0%, and differential solubility was between 0.26% and 2.49%. During the soil incubation, the NH4+-N in soils of all treatments de creas ed; while NO3--N and Nmin in these soils increased. The contents of N H4+-N, NO3--N and Nmin in soil were highest when only conventional urea were added, and low est when only D90 and D60 or their combination were added; the contents of NH4+-N , NO3--N and Nmin in treatments combined conventional urea with CRFs were in th e middle. The NH4+-N in soils derived from different sources during the inc ubati on; in the early 20 days incubation the NH4+-N mainly came from the convent ional urea; and during the 30 to 50 days incubation more portion of NO3--N from D60; and the conventional urea had more important effects on the NO3--N and Nmin in s oil during the whole incubation. As the proportion of conventional urea in the c ombinations of different fertilizers was decreased, the contents of NH4+-N, NO3--N and Nmin in soils also became lower. It is concluded that the mixture design is a useful method to evaluate the nutrient release characteristics of CRFs and their rational application.