The pelvis, a major load carrying component of the musculoskeletal system, is sometimes suspected as a site of mechanically provoked low back pain. Thus, studies of its basic biomechanics seem warranted. This study used biomechanical model simulations to examine how pelvic geometry and joint soft tissue properties influence relative motions among its bones and internal loadings of its joints. A biomechanical model of a pelvis was loaded by forces of up to 1000 N and moments of up to 25 Nm. Its resulting motions and joint loadings were determined. The effects on these responses due to pelvic geometry changes of 20% in lengths and 10 degrees in angles, and soft tissue stiffness changes, most of which were 25%, were also examined. For all situations examined, model pelvis relative displacements were at most a few mm, and relative rotations were at most a few degrees. Internal forces and moments were always less than 530 N and 30 Nm, respectively.