A study was carried out to investigate the effects of side rigidity on passenger injuries in side impacts by using sled tests and mathematical modeling. A multi-body dynamic model was developed to simulate passenger responses in side impacts. The model was implemented using MADYMO program, and simulations were made with different rigidities of crash structures. The simulation results were validated by sled side impact tests. It was confirmed that the pelvis and lower-thorax suffered the most serious injuries during first contact to the car. It was concluded that increasing the side rigidity of the cars could reduce intrusion and improve the protection of the passengers from side impacts.