It is important to investigate characteristics of the vibration of jack up type oil rig foi its successful design. FEM is available to calculate natural frequencies accurately, but there are a few uncertainty to be studied, such as, virtual mass of leg vibrating in water, supporting condition of the sea bed and others. This paper, first of all, tries to find the most suitable model for FEM, and after obtaining experimental virtual mass of leg with model tests, the authors calculated the natural frequencies and the mode shapes by the idealized model composed of beam elements and plate ones, supported elastically at the sea bed. Those calculated natural frequencies in the first three order are found to be in good agreement with the experimental ones obtained by vibration tests using the actual oil rig. Main conclusion are as follows. 1) It may be practically acceptable that virtual mass coefficient of leg composed of circular cylinders, C_V , equals 1.0. 2) On calculating natural frequency of oil rig in jacked up condition, spring effect of the sea bed should be taken into account. 3) In estimating dynamic response of oil rig, the lowest three natural frequencies govern the dynamic response due to wave force. Then the higher frequencies than those can be disregarded for dynamic response.