This study consists of two case studies of landslides in natural slope: Lei Pue Street landslide, Hong Kong and Barabensi Landslide, Nepal. The first landslide transformed into a debris flow while the second one did not. Several numerical models (Slope/W, FLAC, DAN and FLO-2D) have been applied for the analysis of these case studies. The appropriate numerical model and the influential parameters for initiation of gravitative debris flow from initial landslide in natural terrain are identified. The model proposed by Hungr (DAN) has been found to give reasonable results on whether or not a debris flow will occur and on the runout distance if one does occur.