In this study, the effect of nonlinear boundary conditions at liquid surface on sloshing height of cylindrical tank under vertical and horizontal ground motions is discussed in comparison to linear analysis. It is found that this effect mainly depends on both dimensionless sloshing height and dimensionless liquid depth as far as smaller sloshing height is concerned, and simple formula estimating this effect is proposed. On the level of observed large sloshing height due to the 1983 Nihonkai-chubu earthquake, sloshing height calculated under nonlinear conditions are estimated about 10-25% larger than those under linear conditions. Taking into account this effect, response spectra of long period ground motions deduced from observed sloshing height are found nearly equal to two-dimensional response spectra calculated from strong motion seismograms.