Ultrasound is widely applied to clinical purpose of not only diagnosis but also therapy. Also, ultrasound has a potential to be applied for targeted drug delivery with microbubble as a carrier in blood vessel. These techniques require accurate position control of therapeutic ultrasound field based on ultrasound image information. Therefore, we propose a robotic system to control a position of an ultrasound transducer for therapy based on ultrasound image information. The system consists of an imaging probe, a robot with the therapeutic transducer, an optical tracking device to integrate the coordinates and positioning software. The system enables the transducer follows to an imaging probe based on a therapeutic plan. In this study, positioning and following control accuracies were evaluated. The results demonstrate the system has a potential for targeting the transducer based on intra-operative planning.