Abstract: In this paper calculations of the seasonal variation of atmospheric ionization rate in low and middle atmosphere produced by the seasonal variation of atmospheric density in CIRA 1972 are conducted. Cosmic ray ionization source functions are used to numerically calculate the ionization rate in the low and middle atmosphere of the whole globe. The results show that the order of the amplitudes of seasonal functions of the atmospheric ionization rate at altitudes below 70 km are several tens percent. Below the altitude of ionization peak (about 15km), the amplitude as about 10%, the higher the altitudes, the larger the amplitudes, and it will reach 35% at 70 km. Above 20 km the seasonal variations of ionization rates are glossly proportional to that of atmospheric densities, being larger in summer than in winter. Below 20 km, the situation is more complicated.