An elderly occupant model was generated from THUMS Version 4.1 AM50 occupant model by modifying the material properties and geometries of the ribcage to represent those of a 70-year-old (YO) male person and was validated using Post Mortem Human Subject (PMHS) tests. A vehicle frontal collision simulation was conducted, assuming a collision speed of 56 km/h, using the elderly occupant model. The simulation model predicted fractures in the sternum, clavicle and upper ribs. The location of rib fractures was along the seatbelt path. This was consistent with the fracture pattern observed in the PMHS tests. The amount of contact force from the seatbelt to the chest was greater than that from the airbag to the chest, indicating that the seatbelt force contributed more to the rib fractures. As the occupant upper body flexed due to the inertia, the vertical component of the seatbelt force acting on the upper ribs increased. The cross-sectional analysis of the moment inside the ribs suggested that the upper rib fractures were primarily caused by torsion and superior-inferior bending rather than by anterior-posterior deflection, which was commonly thought to be the indicator.