A computational algorithm for 3-D rigid-body collision and its coupling with fluid flow was developed and implemented in FLOW-3D® as an addition to the existing General Moving Object (GMO) model. It is assumed that all the bodies have negligible deformation during collision and instantaneously change velocities when they collide. A set of existing equations of motion for collision under six degrees of freedom were adopted. Modifications were made for collisions of bodies with fixed axis, fixed point and prescribed motion. Numerical methods for collision detection and collision integration were developed. Stronge’s energetic coefficient of restitution was employed to determine completion of collision calculation. The model allows for simultaneous collisions of multiple bodies. Collisions can be perfectly elastic, partially plastic or completely plastic. Surfaces of bodies can be smooth or rough, allowing existence of impulse of friction during collision. Continuous contact between moving objects is modeled through a series of micro-collisions. Several applications of the model with and without presence of fluid flow were made. Good agreements of the computational results with analytical and experimental results were obtained and are presented at the end of the report.