Abstract: Taking a rectangular water tank with horizontal and vertical baffles as an example, the linear damping ratio of the tank subjected to harmonic excitations is derived by the virtual work principle and the energy dissipation principle. Parametric studies are conducted by considering the effects of some influencing factors on the estimated damping ratios including the width and locations of both the vertical and horizontal baffles, water depth ratio and excitation frequency ratio. Meanwhile, the numerically estimated damping ratios are compared with some experimental results from other researchers. The comparisons show that the tank damping ratio estimation formulas derived from the energy dissipation principle and the virtual work principle are consistent under linear conditions, and good agreements are achieved between the proposed numerically estimated and experimental results. The damping ratio of the tank is also found to be significantly affected by the excitation frequency ratio. The research results from this paper would be helpful to estimate the damping ratio of rectangular water tanks with various types of baffle arrangements.