For Advanced Driver Assistance Systems (ADAS), lots of researchers have been constantly researching various devices that can become the eyes of a vehicle. Currently represented devices are LiDAR, camera, and radar. This paper suggests one of the operation processes to study the radar, which can be used regardless of climate change, weather, day or night. Thus, we propose a simple and easy calibration method for Multi Input Multi Output (MIMO) radar to guarantee performance with the initial calibration parameters. Based on the covariance matrix, the modified signals of all channels improve the performance, reducing unexpected interferences. Therefore, using the proposed coupling matrix, we can reduce the unexpected interference and generate accurately calibrated results. To prove and verify the improvement of our method, the practical experiment is conducted with Frequency Modulated Continuous Wave (FMCW) MIMO radar, mounted on an automotive.