To study the effect of lap seatbelt engagement relative to the pelvis in a collision due to the orientation of the pelvis, we researched the individual differences of potential contributing factors, including human skeletal alignment, pelvic angle and pelvic shape. Additionally, we created human body models which represent those individual differences by modifying THUMS ver.4 and analysed seatbelt and occupant kinematics in a collision. The aims of this study are to research the individual differences of other potential factors, such as pelvic size and flesh thickness, and to analyse the effects on the lap belt kinematics. In particular, it is assumed that flesh thickness is associated with the position of the lap belt. Therefore, the initial position of belt was also measured and the correlation between body shape and lap belt position was analysed. THUMS was modified to the models which represent individual differences of above two factors:​ pelvic size and flesh thickness. The seatbelt kinematics on the ilium were studied by using these models, and the results indicated that both factors affect the belt sliding up over the ilium. Eventually, pelvic angle was found to have the greatest influence among the four factors:​ (a) pelvic angle, (b) pelvic shape, (c) pelvic size, (d) flesh thickness. It is therefore essential to control pelvic angle.