The purpose of this research is to estimate the wall effect of spherical graphite-walled cavity chambers using the Monte Carlo method to establish a 60Co air-kerma standard at the National Radiation Standard Laboratory of the Institute of Nuclear Energy Research (NRSL/INER), Taiwan. For more than a decade, the validity of the wall correction term kwall determined by linear-extrapolation methods has been strongly challenged by the Monte Carlo method. In this paper, one goal was to evaluate in detail kwall for spherical chambers varying with wall thickness (0.1 to 2.5 cm), cavity size (1 to 1000 cm3), and incident photon energy (0.02 to 1.33 MeV). The other goal was to obtain kwall for self-fabricated, spherical chambers and then compare it with the historical values in 2003. A significant increase of 0.3% for air kerma in the 60Co field was expected. The difference of bilateral comparison between NRSL/INER and the Australian Radiation Protection and Nuclear Safety Agency was reduced when the calculated kwall, instead of the original estimated value of kwall, was applied for the derivation of the calibration factor. The NRSL/INER primary standards for air kerma will be adjusted in the near future to reflect the changes in kwall described in this work.