<p>A graphite walled cavity ionization chamber of volume 1002.4&#xa0;cm³is maintained in Bhabha Atomic Research Centre as reference standard for measurement of Reference Air Kerma Rate (RAKR) of brachytherapy sources. In order to establish it as the Primary standard for measurement of RAKR of ¹⁹²Ir and ⁶⁰Co high dose rate brachytherapy sources, the Sensitivity coefficient of this ionization chamber was evaluated by Monte Carlo simulations using the FLUKA transport code. The physical design of¹⁹²Ir HDR brachytherapy source (V2 type, Nucletron) and⁶⁰Co HDR brachytherapy source (Bebig) was simulated in FLUKA. Sensitivity coefficients of the ionization chamber were determined by estimating the RAKR of the HDR sources and the corresponding energy deposition in the chamber. The computed sensitivity coefficient of the ionization chamber was compared with the analytically derived sensitivity coefficient for ¹⁹²Ir and ⁶⁰Co HDR sources and was found to agree within 0.24% and 1.47% respectively. Satisfactory results in an intercomparison with IAEA for measurement for RAKR gave confidence in the work carried out.</p>