Studies were made of the electron density profiles at various stages of atmosphereic evoluiton, with the assumption that the atmospheric composition has changed due to the gradual increase of oxygen while nitrogen level has remained constant. The result of model calculations indictes that when the oxygen content is less than about 1% of the present earth's atmosphere level, the major ions in the F2 region will change from O+ to N+. The maximum number density of N+ ions reaches approximately 107 cm-3 because of the absence of a rapid loss mechanism for N+ (on the assumption that the rate constant for the reaction N++O→O++N is less than 10-11 cm3 s-1). The height of the N+ ion density peak is much lower than the height of the F2 layer peak of the current ionosphere.