A virtual forest modeling approach using SRTM and TM data is proposed. L-systems (a plant modeling language) is used to create the coniferous and deciduous tree models with different LAI (Leaf Area Index) values. Output images of these tree models are planted on the 3D terrain built from SRTM data, via the image-based method of crossed-billboards. TM image pl93r026 is used to generate the land use Classification map and retrieve the LAI map. These two maps control the placement of trees inside every terrain triangle. Several visualization techniques are used to improve the rendering speed so that the viewers can fly over the virtual forests in real-time.