Recently, step gas quenching has been proposed to further reduce quenching distortion of automobile gears, which refers to rapid gas cooling of steel from austenitizing temperature to a point above or below Ms temperature (first step), where it is held or cooled very slowly by reducing the quenching pressure or flow velocity for a specific period of time to achieve homogeneous temperature distribution in the charge (second step), followed by increasing the quenching pressure or flow velocity to get high speed gas cooling (third step) to save lead-time. In this study, a practical method to predict the heat-transfer coefficient distributions of a ring gear treated by gas quenching was proposed, and the numerical simulation of heat treatment was used to determine the optimization condition of step gas quenching after low pressure carburizing. The deformation analysis results indicate that the minimum quenching distortion will be attained by optimizing the start time (or temperature) of second step. By using the optimized step gas quenching condition, the lead distortion of a carburized ring gear was reduced by 46% as compared to that of standard gas quenching which is defined as the gas quenching with constant quenching pressure and flow velocity.