The present study evaluates the longitudinal stability characteristics under the influence of the wing-in-ground (WIG) effect by applying Irodov's criteria. First, the applicability of Irodov's criteria in estimating longitudinal stability for WIG vehicles was confirmed by derivation of the criteria and order estimation prior to usage of criteria. Second, aerodynamic analysis was conducted using the Navier-Stokes solver for the Clark-Y wing and wing-endplate combinations at various heights. Finally, longitudinal stability was evaluated using Irodov's criteria. On analysis, four assumptions were set for the evaluation of longitudinal stability under the influence of the ground effect. The most important assumption is that the stability contribution of the horizontal tail is negligible when the ground effect is strong. Under these assumptions, the computational results satisfactorily demonstrate the stable behavior of the vehicle under the influence of the ground effect when the Clark-Y wing and wing-endplate combinations are used.