In motor-vehicle frontal crashes, occupants often suffer from the abdominal injuries when the lap belt excurses over the pelvic bone, commonly referred to as submarining. Especially, it is well known that the obese occupants frequently get injured caused by submarining due to out-of-position belt fittings. This paper aims to investigate the interaction between the pelvis and the lap belt during a frontal crash event. For this purpose, twelve sled tests on four obese female Post-Mortem Human Subjects (PMHS) and four sled tests on the Hybrid III 50th dummy were carried out. In each test, a 3D motion capture system was installed to track the movement of the pelvis and the lap belt. Moreover, the validated subject specific FE model scaled from the 50th percentile male GHBMC model to fit to obese female PMHS in prior study was also simulated. In order to compare the pelvis kinematics relative to the lap belt quantitatively, the angle between the belt-to-pelvis that referred to as “submarining angle” and the distance of the position of anterior superior iliac spine (ASIS) relative to the position of the lap belt at the midline that referred as to “submarining distance” were used as the indexes for assessing submarining. Although poor restrained pelvis are considered in this study, the result show that the combination of the belt position relative to ASIS and belt angle is the key factor that determines the occurrence of the submarining. Furthermore, the biofidelic FE human body model can contribute to explore and estimate the kinematics of the pelvis relative to the lap belt during the crash event.