The free-air ionization chamber is communicating with the ambient air, therefore, the atmospheric parameters such as temperature, pressure and humidity effect on the ionization chamber performance. The free-air ionization chamber, entitled as FAC-IR-۳۰۰, that design at the Atomic Energy Organization of Iran, AEOI, is required the atmospheric correction factors for correct the chamber reading. In this article, the effect of humidity on the ionization chamber response was investigated. For this reason, was introduced the humidity correction factor, kh. In this article, the Monte Carlo simulation was used to determine the kh factor. The simulation results show in relative humidities between ۳۰% to ۸۰%, the kh factor is equal ۰.۹۹۷۰ at ۲۰°C and ۰.۹۹۷۵ at ۲۲°C. From the simulation results, at low energy the energy dependence of the kh factor is significant and with increasing energy this dependence is negligible.