Robot tractors have been developed to solve the problem of shortage of farmers as well as reducing the fatigue of farmers.A global positioning system (GPS) receiver fusion with an inertial measurement unit (IMU) are popularly utilized for the autonomous navigation of the robot tractors.However, it is still an unsolved problem to select a good position for setting a GPS antenna for the autonomous navigation system.In this study, amethod that can be used to decidea good position to set a GPS antenna was going to be designed.The method was using two real-time kinematic(RTK)-GPSto find the best position in order to get higher accuracy for the sensor fusion of the GPS and IMU.And the methodwas not using the parameters of a tractor, such as mass, size,wheelbase length, track length, etc.Therefore, this method can be generallyused for the wheel type robot tractors.In the experiment, two RTK-GPS antennas were set at the front and rear of a wheel type robot tractor.The horizontal position accuracy of the RTK-GPS is ±0.03 m.A virtual GPS antenna position was calculated from the position of the front and rear GPS antennas.And the virtual position was input to a Kalman filter (KF) for the sensor fusion of the GPS and a micro electro mechanical systems (MEMS) IMU to get the true heading angle of the robot tractor.A high accuracy fiber optic gyroscope (FOG) based IMU was utilized as a reference sensor for the heading angle of the robot tractor.The tests were conducted on a straight and a curved paths in a tough surface field so that to find the best position to set the GPS antenna.The results show that there is small influence to the true heading angle of the tractor from the position of the GPS antenna on the straight path.However, on the curved path, a good position of the GPS antenna can reduce the RMS error of theheading angle from about 10to1 deg with comparing with the heading angle of FOG based IMU.From the results of this study, it can be concluded that the proposed method can decide a good position to set the GPS antenna.And from the selected position, a high accuracyofheading angle can be achieved by the fusion of the GPS and IMU for the robot tractor.