One of the main reasons for using a joint torque sensor in a robot is to measure body forces acting on the system. Especially, the motor actuating torque and the external link torque are important for a flexible joint control. However, it is difficult to measure or estimate those two torques simultaneously with a single joint torque sensor only due to the lack of information. In this paper, Kalman Filter based two-port Body Force OBserver(KF 2-port BFOB) was proposed to estimate two output torques from the motor-side port and the link-side port simultaneously using the two-port system dynamics, the joint angle and the sensor torque from the joint torque sensor based on the Kalman filter. The basic idea of KF 2-port BFOB is illustrated and its performance is verified by implementing the algorithm with disturbance observer(DOB) in the motor actuated one-DOF flexible joint robot. The experiments are executed in the planar situation (under the gravity-free condition) and in the vertical situation(under the gravity condition) respectively to verify the performance of KF 2-port BFOB. The results show something with conclusion