|Natural dams are formed sometimes owing to occurrences of landslides and debris flow. Present study describes the formative conditions of the natural dams based on the results obtained from numerical simulations of landslides and debris flow. Numerical simulations are conducted by means of the method proposed by Takebayashi et.al (2015) as well as by Yamasaki et al. (2018). The method is composed of a depth-integrated two-dimensional form for mass and momentum conservation equations of debris flow body and mass conservation equation of bed sediment together with a resistance formula of debris flow and erosion/deposition formula of sediment. Numerical computations are conducted referring to parameters such as bed slope, equilibrium bed slope, potential erosion depth, physical characteristics of bed and slope material, run-out distance in erosive regions and other geomorphologic parameters associated with the torrent network. A criterion is proposed based on the numerical results for the formation condition of the natural dams and it is compared with field data.