The effect of impactor stage geometry on impactor collection efficiency curves is explored numerically. It is found that the geometry of the impaction stage where the gas is accelerated does not affect the 50% cut size but has a strong effect on the sharpness and shape of the efficiency curve. This phenomenon is explained from the aerodynamic focusing effect of the particles in the nozzle (gas acceleration) section. The flat-plate orifice configuration, commonly used in cascade impactors, tends to focus particles closer to the centerline than the angled nozzle. When the real nozzle configuration is used, the prediction shows a characteristic S shape, in good agreement with the experimental results in the literature.