© 2015 IEEE. Previous studies have shown that a transformer's modal characteristics can be identified by its internal electromagnetic excitations and are related to structural damage. These may be useful for diagnosing the causes of failure modes in power transformers during normal operation. Operational modal analysis (OMA) appears to be a suitable tool for this purpose. In this letter, the feasibility of OMA in identifying transformer modal parameters is discussed with a special focus on the mechanisms of natural excitation. OMA is also applied to a 10-kVA transformer with and without artificially designed structural damage. Good correlation is obtained between the results of OMA and those of experimental modal analysis.