Abstract: Theoretical analysis of dynamic behavior of upright cylindrical tanks with initial irregularities on shell is conducted to investigate the effect of irregularities on natural frequencies of liquid storage tanks. The distribution of hydrodynamic pressure in the tank with irregularities on wall is derived by velocity potential function. The irregularities are considered as the initial displacements of the tank. Based on Hamilton's principle, free vibration equations of tank with initial irregularities on side wall are obtained by means of a discretization scheme in which the shell is modeled by semi-analytical finite elements. Not only the effect of the irregularities on hydrodynamic pressure, but also the coupling effects among circumferential modes are considered. The numerical results show that the influence of the irregularities on certain natural frequencies of the tank is considerable.