Ground relief is a key parameter affects the open forest bidirectional reflectance factor (BRF) estimation result of a Monte Carlo ray-tracing model. In this proceeding, we propose a 3-D ray-tracing simulator for estimating the BRF for a forest with complex terrain relief. Unlike existing models, this model takes into account rugged terrain conditions by using a local normal retrieving approach based on digital elevation model (DEM) bilinear interpolation. A comparison between this model and the well-performing Monte Carlo model FLiES for validation as well as a good correlation between the results of the two models was obtained. Forest BRF estimations for some different terrain relief conditions are derived, and these have reasonable variation according to ground conditions.