Abstract: The basic equation discribing Spread--Finstability with the existance of acousticgravity waves in the ionospheric Fregion are derived and the numerical simulationsfor certain cases are conducted. The results show that the Spread-Fwill occur inlarge-scale wavelike structures under the conbined action of acoustic-gravity wavesand the Rayleigh Taylor instability in the equatorial Fregion. This Spread-Fis aquasi-periodic instability moving at certain velocity and at late;developing stage itwill form plumelike plasma density depletions (bubbles) which can rise well abovethe Fpeak and become tilt. The acoustic-gravity wave vector determines the tiltdirection of bubbles. All these results are in agreement with the main observationalcharacteristics. It is thought then that acoustic-gravity waves are not only one ofthe initial sources of large-scale Spread-F, but also play an important role on itswhole evolutions.