Abstract: The free air ionization chambers are used in the primary standard dosimetry laboratory (PSDL) for obtaining air kerma of X-rays all over the world. In this paper, the electric field distribution inside the free air ionization chamber was investigated by finite element method. The study result shows that maintaining the uniformity of the internal electric field distribution by adding a guard electrode and several guard rings is available, and the potential of the lead box can change the electric field distribution inside. The result of the electric field simulation shows that the electric field distribution in free air ionization chamber is optimized applying a suitable potential to lead box, guard electrode and guard rings, and optimizing the structure of the free air ionization chamber.