This paper describes mechanical design for a minimally invasive surgery robot, the robot will help surgons to realize high quality and precise localization in radio frequency ablation for patients with liver tumors. At present, radio frequency ablation minimally invasive more and more using in the minimally surgery operation, therefore, to develope a medical robot assisting surgons to achieve success in the radio frequency operation is necessary. In this paper, we analysis the radio frequency operation, according to the requirement of the minimally invasive operation, a medical robot structure is designed. We discuss the robot work space, gearbox mechanical and robot arm design. through localization experiment shows that the robot precise meet the minimally invasive operation requirements.