Many future applications of robotic systems will require manipulators to operate from moving vehicles. However, different from the manipulators fixed on the floors, mobile robotic systems (vehicle-mounted mobile manipulators) might be unstable and even tip over. This work considers that the stability of such a mobile manipulator has a close relation with the vehicle's motion, the manipulator's posture and motion, and the endpoint's force. This paper presents the concepts of the stability degree and the valid stable region based on the ZMP(Zero Moment Point) criterion, which can be used as effective stability criteria in controlling mobile manipulators. Finally, the concepts are illustrated by computer simulation.