Abstract: A real-time estimate method of wheel-terrain geometric contact angle for all-terrain mobile robots is studied in this paper. For a six-wheeled all-terrain mobile robot with a passive-compliant mechanism, the wheel-terrain geometric contact angle δ is defined to reflect the effects of terrain profile and the contact relationship between the driven wheel and the terrain. The robot is treated as a series-parallel multi-rigid body system. The kinematical model of the robot body considering the unevenness of the terrain and wheel slips is developed based on closed velocity chain. Since δ is hardly measured directly by on-board sensors, this paper proposes a model-based estimate method by using common on-board sensor data to on-line estimate the contact angle δ . The Kalman filter is used to fuse noisy sensor data. The experimental results demonstrate the availability and effectiveness of this method.