Studies about a mobile robot have been developed. Especially, there have been a lot of theories about the trajectory planning of a mobile robot or noholonomic constraints or tracking controller. But it has not argue what posture is suitable for a mobile robot, because the index to decide the condition of a mobile robot has not been proposed. In this paper, "mobility of a mobile robot", is defined applying the notion of manipulability using "mobility Jacobian matrix" in order to evaluate the posture of a mobile robot. At first, two mobile robots, which are an actual posture and a desired posture, assume to be rotational motors and a stationary motor. These motors presume to be connected by a virtual link between two robots, so that two mobile robots presuppose to be a virtual manipulator. Then, mobility Jacobian matrix related in the velocity vector of an end effector and rotational velocities of wheels is defined like Jacobian matrix of a manipulator. The results of simulations show the validity of the proposed definition and are prospected.