By using the equations of kinetic and available potential energy in the wave-number domain, mechanism for the maintenance of the zonal mean flow and ultra-long waves at the 200-hPa level over the tropics from June 11 to August 13,1983, is analysed with the ECMWF/WMO data. The result shows that the tropical easterlies in summer are barotropically stable. The waves are found to transfer kinetic energy to the mean flow. In the interaction between the various waves, the wave of wavenumber 2 plays the role of an energy source. The work done by the lateral pressure force and the lateral convergence are also important in the maintenance of the ultra-long waves in the tropics.