In RCAR bumper test, the deformation of bumper beam is influenced by bumper engagement. In this test, it is required for bumper beams both to increase energy absorption and to reduce the vehicle weight by designing cross sectional geometries adequately. In this study, the deformation mechanism of bumper beam has been investigated from the view point of the effects of bumper engagement and cross sectional geometries, using numerical analysis. As a result, it is shown that bumper beam with small radius of impact side and wall angle θ_w>90 degrees absorbs large energy in this test, because buckling of cross section is suppressed by the effect of restraining bending deformation of impact face and wall.