A free-air ionization chamber FAC-IR-۳۰۰, designed by the Atomic Energy Organization of Iran, is used as the primary Iranian national standard for the photon air kerma. For accurate air kerma measurements, the contribution from the scattered photons to the total energy released in the collecting volume must be eliminated. One of the sources of scattered photons is the chamber&#۳۹;s diaphragm. In this paper, the diaphragm scattering correction factor, kdia, and the diaphragm transmission correction factor, ktr, were introduced. These factors represent corrections to the measured charge (or current) for the photons scattered from the diaphragm surface and the photons penetrated through the diaphragm volume, respectively. The kdia and ktr values were estimated by Monte Carlo simulations. The simulations were performed for the mono-energetic photons in the energy range of ۲۰ − ۳۰۰ keV. According to the simulation results, in this energy range, the kdia values vary between ۰.۹۹۹۷ and ۰.۹۹۴۸, and ktr values decrease from ۱.۰۰۰۰ to ۰.۹۹۶۵. The corrections grow in significance with increasing energy of the primary photons.