In order to obtain the response of structures subjected to dynamic loading knowledge of the free vibration characteristics of the structure is essential. The enclosed fluid in the tank exerts inertial loading on the structure, and part of the fluid may be coupled to the motion of the structure. For partially filled tanks the free surface motions of the fluid may be coupled with the tank motion, further complicating the analysis. In this paper, free vibration behaviour of a fuel tank was determined by numerically. Parametric studies have been done in order to see which parameters affect the vibration behaviour of the tank.