Abstract: In this paper, variations of the main ionospheric parameters during large earthquakes are analysed and discussed. These parameters are the electron content below the ionospheric peak, electron density at the altitude of 200 km and the height of the maximum electron density etc. The variations of other ionospheric parameters are also analysed. The existence of the ionospheric disturbances has been well verified before earthquakes, and this paper provides some preliminary information for the prediction of an earthquake.