Abstract: Based on nonlinear-sliding mode control,motion control of the under-actuated spherical robot is studied.Inputs of the spherical robot consist of two perpendicular torques.With nonholonomic constraint,kinematic and dynamic models of the spherical mobile robot are derived,and with the help of input transformation,the dynamic model is transformed into a second-order system with two inputs.Based on nonlinear sliding mode control,lateral attitude controller and longitudinal ve- locity controller are designed respectively to enable the controlled states to converge to the desired neighborhood.Simulation and experiment results confirm the validity of the presented dynamic model and control algorithm.