High reliability is essential for power transformers, and their fault causes are reportedly more related to mechanical causes than electrical ones. The transformer soundness judgment currently depends only on the electrical insulation characteristic and the chemical test of the insulation oil, so that there are few fundamental measures against the frequent mechanical damages and failures in transformers. The mechanical soundness judgment techniques are conducted through processes that include structural analysis and vibration resistance treatment during the manufacturing process of each manufacturer, but the vibration is not tested during the design, manufacturing, and operating processes since there are no detailed technical standards and procedures on the vibration problem, which are important in terms of maintenance. Therefore, in this study, vibration phenomena were measured from the 32 power transformers in operation in the substations under the Daejeon Power Transmission District Office of the Korean Electric Power Corporation (KEPCO). The vibration was measured at 24 sections ($6{ imes}4$) on one side, and only the maximum values were selected from the measured vibration values. This was because the maximum vibration values more significantly affect the soundness of the transformer than the average vibration values. The vibration classes were given considering the maximum vibration based on ISO 10016-1 (2001).