Abstract: The effects of the cold fronts upon the night f₀F₂ are found out by using the records from the cold fronts years book and ionosonds measurements of Wulumuqi and Changchun. From the statistical analyses of large number samples from 1968 to 1975 and the superposed epoch analyses with the day of the minimum temperature during the processes as the key day, it is shown that the f₀F₂ tends to decrease when a could front passes across. The maximum decrease is at the key day in the superposed epoch analyses. The pereentage values of the decreases are 4%-8%. It is larger in solar low-activity years, and the variation is also larger in Wulumuqi than in Changchun, x²-and U-test have been used to demonstrate the significance of the respose. The effects are probably caused by the gravity waves from the Jet-stream?generated Kelvin-Helm-holtz Instability within the cold front area. Gravity waves may propagate upward and produce the up-down oscillation in ionosphere. The integrated decrease of the f₀F₂ and the difference between the high and low activity years can be interpreted by his mechanism as well as the station-to-station difference.