Abstract: The goal of this paper is to develop an ultrasound-guided robot system of microwave coagulation therapy for liver cancer which consists of 3D image navigation, needle-driven robot and electro-magnetic tracking unit.In the pre-operative phase,the surgeon reconstructs the 3D model of liver tumor and plans the needle insertion path with this system.In the intra-operative phase,to transform the positions in pre-operative model into the joint angles of robot,the surgeon matches the pre-opertive 3D model of liver with real patient liver by the blood vessels in liver.At last,the surgeon accurately inserts the needle to the target point by moving the robot system to destroy the tumor with microwave coagulation.In phantom experiment,the error of the system is no more than 5 mm,which can meet the need of microwave coagulation therapy for liver cancer and improve the efficacy of liver treatment.