Rockfalls are frequent instability processes in road cuts, open pit mines and quarries, steep slopes and cliffs. The attitude and persistency of joints within the rock mass define the size of kinematically unstable rock volumes. Furthermore, the rock block will eventually split in several fragments during its propagation downhill due its impact with the ground surface. Knowledge of the size, energy and trajectory of each block resulting from fragmentation is critical in determining the hazard of buildings and protection structures. The objective of this study is to simulate stochastically the fragmentation mechanism in rockfall propagation trajectories and in the calculation of impact energies in a GIS-Based tool which includes common modes of motion for falling boulders. A stochastic fragmentation model is proposed and tested to determine if it can simulate the fragmentation phenomena properly.