The letter deals with theoretical analysis on Lamb waves propagating in a composite membrane of ZnO and SiO2. Calculation was done for the lowest pseudo symmetric (S0) and antisymmetric (A0) modes. It was shown that, for example, when the thickness of each film is of the order of 0.1 wavelengths or less, S0 mode has a large electromechanical coupling coefficient of about 6% and a velocity of more than 5,000 m/s. A0 mode, as may be expected, has a quite small velocity of less than 1,500 m/s with an electromechanical coupling coefficient of about 3%. The result suggests that it may be possible to realise various monolithic acoustic wave devices based on Lamb waves propagating in the ZnO/SiO2 composite membrane.