The impact-induced rock mass fragmentation in a rockfall is analyzed by comparing the In Situ Block Size Distribution (IBSD) at the cliff face and the Rockfall Block Size Distribution (RBSD) on the slope. The IBSD is extracted from a Discrete Fracture Network generated using a Digital Surface Model of the cliff obtained with digital photogrammetry. The RBSD is obtained directly by measuring the blocks deposited on the slope. The IBSD and RBSD are well fitted by exponential and potential functions, respectively. We present a procedure to derive the RBSD from the IBSD using a fractal fragmentation model based on Perfect (1997) and considering a survival rate of the blocks impacting on the ground surface.