Abstract: By using a simple air-sea coupled model, the interaction of Rossby waves between the air and sea in the tropics is discussed. It is shown that the coupling of Rossby waves in the two media produces not only the westward propagating waves, but also a type of new wave which moves eastward. The eastward propagating waves exist in the scope of comparatively long wavelengths and this scope is governed by the intensity of the air-sea interaction. In addition, insta-bility may appear in both the eastward and westward propagating waves, and the wave amplifying rates are also governed by the intensity of the air-sea interaclion. In the end, a possible explanation to ENSO events is given in terms of the air-sea interaction waves.