Abstract: An online calibration method based on visual markers is proposed for the service manipulator with joint clearance to solve the drift problem. The marker mounted on the manipulator is recognized and its 6DoF (degree of freedom) is calculated by the stereo vision system firstly, and then the real positions of the joints are computed through solving the inverse kinematics of the manipulator, and thus the drift of joints' readings are estimated to accomplish the online calibration of the manipulator. Along with the property that the drift of joints' readings keeps stable within a short time, the proposed method can make the end-effector precisely reach the target operating position. Experiments of autonomous microwave-oven button pressing on a mobile robot platform equipped with a 5DoF low-cost manipulator with joint clearance demonstrate the validity of the proposed method. And auxiliary experiment validates the assumption of local stability of the joints' readings drift.