A vibration troubleshooting activity was performed on a Centrifugal Compressor due to a high level of vibration (63 µm) during normal operating conditions. The vibration analysis carried out on site led to detect an unbalance source on the centrifugal compressor , and a single plan trim balance was performed. An additional mass of 8.7 gr was installed on coupling flange. By means of this weight correction, the vibration levels decreases from 63 to 17 µm.