The main inﬂuences of plants on the mass stability of slope are those that affect the strength of soil. Plants enhance soil strength by directly reinforcing soil material with their roots. In this paper we do not consider slope hydrology but focus on quantifying increases in plant root growth. Root length is a function of growth time. In this paper, the sigmoidal models have been used to analyze plant root growth with time. Field studies of slopes of YU Tong road , two kind of tree, weeping willow, alamo and three kind of plant, Tall fescue, Bermuda grass and White clover indicate that slope reinforcement, due to the roots of these species, increases exponentially with time below the soil surface. Differences in the root length of tree and plant are illustrated by Logistic model, Richards model based on measured data.The measured data are a large scattering and is diffcult to find growth trend and predict the final length in one season. The data must be process before used in practical engineering. Sigmoidal models are a good tool to determine the growth-time curves of root length. For this purpose, two different sigmoidal models were used to define the curves of root growth, namely Richards model and Logistic model. The coefficients of Richards model for plant were 0.9964591, 0.998516, 0.9964465 and Logistic model for tree were 0.984001, 0.9957275 respectively. The analysis results show that Logistic model, Richards model are suitable models for explaining root growth.