Coupling between atmospheric gravity waves and plasma interchange instability in the equatorial ionosphere is studied. The interchange instability is saturated at a small amplitude by nonlinear effects if there are not any external geophysical distu-rbances to influence it. It is shown that the gravity waves can initiate the interchange instability under some conditions. The initiated instability grows to an amplitude of 50% even larger and forms plasma bubbles if the amplitude of the seed ,gravity waves is large enough. The present theory shows physical mechanism for producing large scale equatorial spread F and explains a lot of ionospheric observations.