Abstract: According to the phenotypic characteristics of life-cycle and fluctuation with parities, four mathematical models including Poisson cycle model, cubic polynomial model, modified quadratic polynomial model I and modified quadratic polynomial model II were respectively used to fit the expected litter size at different parities for Jiangquhai sows. From the viewpoints of statistics and biological meaning, modified quadratic polynomial model I was the optimum model of litter size. Single trait animal model and DFREML procedures were involved to estimate the heritabilities for optimum model parameters. The results showed that the values of heritabilities for model coefficients A and B and the estimation of acmes of pure quadric curve in model were higher than that for litter size, which hinted that selection on model parameters should be more effective than direct selection on litter size.