By using a general circulation model, excitation mechanism of intraseasonal oscillation of the zonal mean zonal wind in the equatorial mesosphere and lower thermosphere (MLT) is investigated. Our results show that wave-mean flow interaction with ultrafast Kelvin waves and diurnal tides is important for driving the intraseasonal oscillation. Not only the migrating diurnal tide but also the nonmigrating diurnal tide plays an important role in excitation of the intraseasonal oscillation. Intraseasonal variations of amplitudes of the ultrafast Kelvin waves and the diurnal tides are found at height range from 20 to 90 km heights, indicating dynamical coupling between the MLT region and the lower atmosphere. Intraseasonal variations of amplitudes of these waves and their relations with the tropospheric intraseasonal oscillation are discussed.