Abstract: Using the data of 100 kHz low frequency radio wave (Loran-Csignal), the nighttime integral electron density in the ionosphere below 100 km at low latitude are calculated, and the cause of this variations are analyzed under various magnetic disturbances during 1986-1987.The results show: In the lower ionosphere at low latitudes, the nighttime variations of electron density are related to geomagnetic disturbances. The electron density are smaller, and changing following geomagnetic disturbance in one or two days during geomagnetic quiet days. The electron density are larger during and after magnetic storms. The fluctuation of the integral electron density in this region can be one order greater, it reaches peak value in 2-3 days after a magnetic storm. The bremsstrahlung radiation produced by the precipitating electrons may be a main factor which causes the nighttime variations of electron density in the lower ionosphere at low latitudes.