This research presents a running model for a compliant wheel-leg hybrid mobile robot. A wheel-leg consists of three legs arranged by 120 degrees to each other and passive compliant joints. For simplicity, each leg is treated as a linear spring such that a traditional mass-spring model can be applied to model running of the wheel-leg hybrid robot. In order to achieve stable running, we propose a simple controller that can converge a robot leg to a desired angle of attack asymptotically during the swing phase. Hybrid wheel-leg running is then simulated and results are discussed.