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Searching for optimal needle insertion path by intraoperative magnetic resonance imaging (MRI) is complex and time-consuming task in MR-guided microwave thermotherapy of liver tumor. This paper reports the semi-active needle-guiding robot for MR-guided therapy by which a surgeon can freely selects needle insertion path while maintaining the remote center of motion at the pre-defined tumor site. The robot consists of three-degree-of-freedom active XYZ base stage with ultrasonic motors and passive needle holder with unconstrained two-degree-of-freedom rotation measured by optical fiber sensor with aluminum die cast. Once the target point on the tumor is defined in volumetric MR images, the base stage keeps the remote-center-of-motion at the target point, while physician can interactively selects the orientation of the needle path with continuous single slice MR imaging. Experiments to test the accuracy of remote-center-of-motion indicated that the robot can keep the needle tip with an average error of 0.7 ∼ 0.3 mm in the compatibility test showed the robot can be safely used in MR axial plane, and 1.0-0.4 mm in the sagittal plane. Unlike related studies published elsewhere where the MR compatible robot autonomously brings the tools to predefined location, the proposed approach is fit to use in microwave thermotherapy of liver cancer.