Abstract: Atmospheric gravity waves generated in the lower atmosphere, carrying momentum and energies, propagated upward and usually became saturated and breaking in the Mesosphere and Lower Thermosphere (MLT) due to their increasing amplitudes versus increasing height. Linear saturation parameterizations of atmospheric gravity waves were applied with the β-channel quasi-geostrophic mean flow equations to qualitatively investigate influences of gravity waves on MLT circulations. Three waves with 0m·s^-1, 20m·s^-1,-20m·s^-1 horizontal phase speeds respectively were taken as a parameterization of the discrete spectrum of gravity waves. Simulations show that the accelerations due to waves can be 100m·s^-1/d and eddy diffusion coefficients can be 120m²/s in the mesosphere, which balance the Coriolis force due to MLT meridional circulations and induce great departures of the zonal mean zonal winds from the radiation balance zonal winds. The calculations of the zonal mean zonal winds of MLT are qualitatively consistent with the MF radar observations and model profiles both in winter and summer time.Resultsshow gravity waves play significant role in MLT circulations.