Abstract: The factor K_wall to correct for photo attenuation and scatter in the wall of the 30 cm³ and 10 cm³ spherical chambers at National Institue of Metrology for ⁶⁰Co air-kerma measurement is determined by Monte-Carlo calculation and experimental measurement. The calculation using the EGSnrc software system shows that the calculated wall correction factors differ by up to 1.0% from those obtained by linear extrapolations of the measured relative ionization current to zero wall thickness for the measured chambers. The relative ionization current and wall correction factors for the 30 cm³ and 50 cm³ spherical graphite chambers at NIST are calculated and compared with the measured results. The Monte-Carlo code can reproduce the experimental relative ionization current within an accuracy of ±0.12%. This work confirms that for spherical standard chambers the ⁶⁰Co air-kerma reference values should be obtained by a new K_wall determined by Monte-Carlo calculations.