Despite that omni-directional mobile robots have been employed popularly in several application areas, effort on optimal design of such mobile robots has been few in literature. Thus, this paper investigates the optimal design of omni-directional mobile robots. Particularly, optimal design parameters such as one or double offset distance of wheel mechanism and the wheel radius are identifed with respect to isotropic characteristic of mobile robots. In addition, the force transmission characteristics and actuator-sizing problem of mobile robots are investigated Analysis has been performed for three actuation sets. It is shown that the redundantly acruated mobile robot with three active caster wheels represents the best performance among them.