Abstract: Using data of nine ionospheric stations of China during Dec., 1975-Dec., 1986, it is presented that under the conditions of certain latitudes and solar activity, night time f0F2 shows a seasonal anomaly similar to that of the daytime foFz. This anomaly obviously shows two features: it only occurs in low latitudes; the higher solar activity is, the larger exlent of the degree of the anomaly is to. The night time seasonal abnormal phenomenon is the continuation of daytime seasonal abnormal phenomenon. The promary mechanism of the phenomenon is the seasonal variation of neutral air (O, O₂, N₂) concentration ratio O/O₂ and O/N₂, caused by the summer hemispher to winter hemisphere air flow. It is analysed why the night time seasonal abnormal phenomenon is limited in low latitudes and why the abnormal degree changes with solar activity.