In molecular dynamics (MD) simulation, large-scale density fluctuations bring about difficulties in evaluating the critical point from coexisting vapor-liquid densities. This study proposes a gravity perturbation method that improves the stability of vapor-liquid coexisting phases near the critical point by introducing gravity and boundary walls. Trial simulations were performed with a modified flexible SPC-TR (Toukan-Rahman) model and a critical point obtained for water (T-C = 377.6 degreesC, rho(C) = 0.302 g cm(-3)) was in good agreement with the experimental data (T-C = 374.0degreesC, rho(C) = 0.322gcm(-3)). The method was also applied to NaCl-water mixtures and it was found that simulations were greatly facilitated with the technique. The gravity perturbation method allows reliable determination of phase behavior in the vicinity of a critical point.