Abstract: VR applications on the Web demand lightweight virtual models and scenes. Among the existing tree modeling methods, the rule based modeling method, e.g. L-systems or automatons, should be the lightest. This paper presents an interactive approach for users to model trees easily. By adopting the spherical coordinate system, the modified turtle geometry provides a more natural interpretation when modeling trees. Based on a modified version of parametric L-systems, our approach allows users to modeling trees in a What-You-See-Is-What-You-Get manner. Both theoretical analysis and experimental results show the effectiveness of our proposed solution to lightweight Web3D tree modeling, thus it becomes feasible to construct large scale virtual forest scenes on the Web.