Accurate load sensing is increasingly important in performing various tasks involved with contacting the environment. Most research has been focused on improving the hardware of a force/torque sensor. Torque sensors for a manipulator suffer from crosstalk, which cannot be compensated even by calibration. Thus they require precise machining and complicated structure to minimize crosstalk, which often results in cost increase. This research proposes a new calibration method composed of two steps. By the primary calibration, the torque sensor output can be related to the joint torques. The secondary calibration, which is based on the virtual load, is conducted to compensate crosstalk of the torque sensor. The virtual load is obtained from the measured joint torques and the manipulator Conf.iguration. Using the proposed calibration method, the external load acting on the end-effector can be sensed accurately even with relatively low-quality torque sensors. Experimental results showed that the error in load sensing was significantly reduced by the proposed calibration method.