The inseparable parts of any industrial unit are usually the rotary machines. Fans are categorized as a common type of rotary machines, which play an important role in the industry. In order to decrease the repairing costs and energy consumption, Fans have to operate without vibration. However, if the fan unit with an acceptable level of vibrations is installed on a huge structure, the vibration caused by the fan can develop complications for the structure as well as serious problems for the fan itself. A long-time operation of a faulty fan can cause failure in the fan motor and fatigue in the structure. Therefore, investigating the root causes of the vibrations of the fan and decreasing the vibrations is vital for increasing the operating time and the efficiency of the fans. This study is focused on identifying the root causes of excessive vibrations of one of the air fans in BualiSina petrochemical company. First, the main frequencies which are responsible for the increase in vibration levels are identified, by using ODS analysis. Then, the natural frequencies of the structure are derived using the operational modal analysis (OMA). Also, the finite element model of the fan unit and the structure is developed based on the most possible compatibility with the experimental data. Finally, a number of suggestions for reducing vibration amplitudes of the fan are proposed.