Liquid storage tanks as special structures should be exploitated after ground shaking, because these structures are used to inhibit the fire and supply the urban water. So, plenary knowledge of the seismic performance of such structures is necessary to supply their safety and decrease the retention costs. In this study, the effects of baffle plates in different sizes and positions are investigated. Using the boundary element method the Laplace equation is solved in the rigid tank in both frequency and time domain. Then, the effect of baffle plate on the sloshing natural frequencies, sloshing height, base shear and overturning moment of liquid tank subjected to harmonic shakings, elcentro and kobe earthquekes ,are investigated.