Research on the functional relationship between leaves and conductive tissues of trees is very important, as it can lead to new knowledge on tree growth and life. Although previous studies have examined leaf expansion qualitatively, quantitative estimation of leaf expansion plays an important role in analysis of physiological leaf phenology. So far, three curves, namely the Mitscherlich, logistic, and Gompertz curves have been applied to best fit the time course of leaf expansion, while Richards’ growth function has not been previously evaluated. This function has five parameters (A, k, b, m, and t0), and comprises the above mentioned three types of curves which differ regarding the ‘m’ parameter. This study aimed to estimate time courses of changes in leaf area using Richards’ growth function and to develop leaf expansion model, which can be used in common without depending on a difference of tree ring porosities. Leaves were sampled periodically from ring-porous jolcham oaks, Quercus serrata, and from diffuse-porous American sweetgum trees, Liquidambar styraciflua, both of which grow in temperate forests. Richards’ growth function was able to describe seasonal changes in leaf area in both species, and an asymmetrical function of the Gompertz was suitable for them. We suggest that Richards’ growth function can be used to estimate leaf area time courses without concern about the curve type, and can further be used to calculate leaf expansion quantitatively.