A surgical navigation system designed for percutaneous abdominal therapies was proposed and evaluated in clinical applications. In addition to segmentation and 3-D display of tumors using MRI, its integration with US can help surgeons deal with instabilities such as respiratory motion and soft tissue shift that are inherent in abdominal surgery. We also included a process to correct the registration error between MRI and US resulting from patient posture change or marker shift. The correction matrix computed by measuring the needle position in the MR and US images is multiplied by the registration matrix. Phantom and animal experiments demonstrated that the proposed method could combine the advantages of both MRI and US. The average insertion accuracy was less than 3 mm, and the time required to determine the optimal insertion path was less than that required under either US or MRI guidance alone. The newly developed system was applied to two patients who underwent PEIT therapy, and its feasibility in clinical applications was verified.