A robot driven by a McKibben pneumatic actuator generates stable motion despite its simple control and simple actuator model. However, how its characteristics act on the stability of robot motion have not been sufficiently discussed. In this paper, a model of the McKibben pneumatic actuator is derived in which the actuator characteristics are considered. In modeling the McKibben pneumatic actuator, we concentrated on its dynamic characteristics.We applied the actuator model to a simple robot model and analyzed the stability of the motions generated by the actuators. From the stability analysis results, we found that the stability of the constant posture is satisfied by the static and dynamic characteristics of the McKibben pneumatic actuator.