In this study, the collection efficiency of a round-nozzle inertial impactor was enhanced through use of an additional punched impaction plate. The additional plate was installed between the nozzle outlet and the existing impaction plate. To determine its effect, a simulation and an experiment were conducted. A parametric study was performed on a PM2.5 inertial impactor to improve the optimum efficiency based on the hole diameter of the additional plate and its distance from the existing plate. As a result, the cut-off size was reduced by 27% from 2.52 µm to 1.84 µm. The effect of the additional punched impaction plate on PM1 and PM10 inertial impactors was also investigated. The improvement in efficiency could be observed even when the additional punched impaction plate had varying Reynolds numbers and nozzle sizes. Accordingly, the (Stk50)0.5 value was reduced by 25% from 0.49 to 0.37 by adding a punched impaction plate in a round-nozzle inertial impactor.