Abstract: Active experiments in the ionosphere, which give rise to artificial disturbances of electron density, are usually conducted by the injection of chemical clouds at ionospheric altitudes. Their effect has often been called an ionospheric hole simply to dramatize the localized nature of the disturbances, which had been observed in many experiments. For example, the injection of rocket exhaust neutral molecules into ionosphere, predominantly H₂O and H₂, can result in ionospheric electron density depletion. Main physical mechanism in these experiments is due to ion and molecule exchange reactions, which enhance the effective rate of recombination of electron. Ionospheric disturbances produced by artificially chemical releases have many applications in radio propagation and exploring ionosphere and plasma physics study. In terms of diffusion process of neutral gas and relevant chemical reactions in terrestrial ionosphere, the theoretical model of ionospheric modification created by active chemical releases is investigated in this paper. The influences of injected H₂O and SF₆ gas on the ionosphere under exponential atmosphere background are theoretically calculated, respectively. The results show that diffusion process of the injected neutral gas at ionospheric height is very quick, and the two released chemical materials can produce localized electron density depletions in F-regions, and those released materials, which have slow diffusion speed and large chemical reaction rates, will substantially affect ionosphere, and facilitate formation of artificial ionospheric hole.