Abstract: To solve the motion control problem of wheeled mobile robots in round ducts or pipes,kinematic characteristics of wheeled mobile robots in round ducts or pipes are analyzed.Description of the position and motion of a wheel on a plane is applied to the cylinder of round ducts/pipes by dint of the tangent plane of the contacting point.The trajectory and velocity of the center of the wheel is educed when it purely rolls in the cylinder.With the theory of instantaneous screw of a rigid-body motion,the kinematic model of three-wheel type(1,1) robots with two fixed wheels and a steering wheel in the cylinder of round ducts/pipes is presented and discussed.Simulation results are presented to show the performance of the kinematic model.