Abstract: The purpose of this study is to gain a better understanding of the distribution of ultra-long waves in low latitudes. The behavior of the ultra-long waves, wavenumber 1-3, is investigated in terms of vertical and horizontal Normal modes decompositions.In this paper, the primitive equations on the β-plane in low latitudes are described. A latitudinal mean flow containing the easterly shear is considered as a basic state. Firstly primitive equations are linearized, then decomposed. Horizontal structure equations are resolved by Normal modes. The results show that the ultra-long waves exist in the upper troposphere and lower stratosphere. Rossby wave is the main element which affects flow field in low latitudes.Secondly, the primitive equations with idealized heat sources in low latitudes are discussed, and solved numerically. The results show that the forcing of the heat sources in low latitudes is one of the factors forming ultra-long waves in the upper troposphere and lower stratosphere.