In the following paper, the result of the in-situ investigation of a rockfall event in the Lien-Tung area of Northeastern Taiwan is used to describe the simulation approach of rockfall trajectories using mathmatical models and computer program. The in-situ investigation was conducted in order to obtain parameters for the simulation. They include source locations and failure type of boulders, rockfall path and footprints, shape and size of boulders, slope inclination and length, slope surface roughness, and slope geomaterial properties. The trajectories of rockfall were back-analysed by using mathematical models and Colorado Rockfall Simulation Program. This analysis can be used as a tool to study the rockfall trajectories needed for rockfall mitigations.