A theoretical study was conducted to prove that the Part 572 dummy submarines more easily than human cadavers because of their different pelvic shapes. The three-dimensional geometrical definition of the lap-belt was developed in this paper for the study of the submarining problem. It was found that not only the lap-belt angles from side view (β1) but also from the upper view in respect to the X-axis (β2) play a significant role in the submarining tendency for each side. The anti-submarining scale is defined by a coefficient which is a function of both angles β1, β2 and the orientation of the upper half of the pelvic notch.A series of sled tests was performed on human cadavers, the Part 572 dummy and the modified dummy. Good agreement was found between the present theory and the experimental results. Some of the experimental results cannot be explained with the traditional geometry definition of the lap-belt given in two dimensions, but they are clarified with the present definition given in three dimensions.The graphs of the anti-submarining scales can be used as tools for checking the risk of submarining tendency in the geometrical design of the seat-belt system.The evaluations of the Part 572 dummy with modified abdominal tissue and pelvic shape at the notch are also presented: the submarining tendency, indicated by the relative occurrence number and the, instant, are achieved more simulatively in the modified dummy than in the Part 572 dummy with respect to the human cadaver.