A three-dimensional model of root system development and soil water flow is described and applied to actual conditions along a hillslope. In the model, gravitropism, hydrotropism, and circumnutation were employed as the main factors controlling root elongation. Root systems of 2-yr-old pine trees (Pinus massoniana Lamb.) on natural slopes in southern China were excavated and examined, and their development was simulated through the use of continuously monitored temperature and rainfall data. In the simulated root systems, angles between first-order lateral root segments and the vertical direction on the upslope portion of a tree were larger than those on the downslope portion of the tree; hence, root systems exhibited asymmetric architectures. This asymmetry was more obvious for root systems developed on the downslope side of the hillslope. Because root systems simulated without the effect of hydrotropism did not develop asymmetric architectures, the direction of soil water flux and the effect of hydrotropism appear to be the main factors contributing to the observed architectural asymmetry, typical of root systems along hillslopes. Calculations with the proposed root system model were helpful in elucidating and understanding the predominant processes affecting root system development on hillslopes.