To get an insight into the dynamic behavior of the dynamically loaded internal gear motor/pump, the crucial analysis problem is prediction of the ring gear motion (orbit) inside the housing under arbitrary loads. The aim of this paper is to study the start-up (zero load) and the dynamically-loaded behavior of the ring gear in the internal gear motor/pump. The theoretical mobility method is used to predict the ring gear movement during its start-up and dynamically-loaded cases. A test rig was designed and built to validate the simulation results. The theoretical and experimental results have a reasonable agreement. The results also point out that the strong vibration of the ring gear will occur when the initial eccentricity of the ring gear is large. Moreover, the orbit, the pressure distribution, and thermal behavior of the fluid film are also analyzed.