The calculation of the ionization chamber response is one factor in establishing a primary standard of air kerma in Thailand. The EGSnrc Monte Carlo code was used for calculation a cylindrical model of the ionization chamber. The chamber was computed the response by applying the Cs-137 spectrum incident on the chamber flat surface. The physical constants simulation comprises the stopping power ratio between graphite and air, the ratio of mass-energy absorption between air and graphite, the fraction of electron energy lost by bremsstrahlung production in air. The constants were calculated by a radiation key data recommendation follow ICRU report 37 and ICRU report 90. Moreover, Wall scattering and attenuation correction and axial non-uniformity were also calculated. The results base on ICRU report 37 are 1.0100(11), 0.9992(6), and 0.0012(2) for the stopping power, the ratio mass-energy absorption and electron energy loss, respectively. However, the stopping power ratio follow ICRU 90 is decreased by 0.78%. For the correction factor, wall scattering is 0.9623 (1) ,wall attenuation is 1.0671 (1) ,and 0.9998 (5) for axial non-uniformity. The calculation results were compared with PENELOPE code and EGS5 code. The comparison between this work and the others is not a significant difference. Therefore, the result can be accepted to use for the graphite cavity ion chamber.