Abstract: In this paper, the behavior of the electron density between the height of 66 and 80km observed by MF radar at Waknnai station （45.36°N, 141.81°E） during four X-class flares is studied. The sudden increase of the electron density is observed during all of the flare period. The results show that the MF radar can be used as one routine method to observe electron density of the ionospheric D region. By comparison with the soft X-ray flux, it is found that the temporal variation of the electron density in the lower D region has a good correlation with the soft X-ray flux observed by GOES. Nevertheless, the correlation is not good in the higher altitude because of the atmospheric disturbances in the D region. On the other hand, the increases of electron density depends on the parameters of these four flares, the increase of the electron density for some of flare reach as high as 400 cm^-3, some of them is only about 100cm^-3. Compared with the increase of the total electron content （TEC） caused by the flare with similar parameters, the total increase of electron density between the height of 66 and 80 km is only about one thousandth of TEC that almost has no effect on the delay time of the radiowave. But, the effect of the Sudden Ionospheric Disturbances caused by the increase of the electron density is very strong due to the very high number density of the atmospheric neutral compositions in the height of D region. In the end, the effective recombination coefficient at different heights is estimated using the electron density at the recovery period during the flare on Nov. 4, 1997.