This study has investigated the influence of impaction plate diameter (Dc) and particle density on the particle collection efficiency of single round-nozzle inertial impactors numerically assuming incompressible flow. The study shows that computed √St50 values range from 0.473 to 0.485, which are nearly independent of W/Dc (W is the nozzle diameter) for the nozzle Reynolds number, Re > 1500, and when W/Dc < 0.32. √St50 values agree quite well with the theoretical values of Rader and Marple (1985), 0.49, and Marple and Liu (1974), 0.477. For a smaller impactor plate diameter such that W/Dc > 0.32, √St50 will increase slightly. It increases from 0.483 to 0.507 (Re = 3000) or from 0.479 to 0.495 (Re = 1500) when W/Dc is increased from 0.32 to 0.48. When the nozzle Reynolds number is smaller than 1500, the influence of W/Dc on √St50 is found to be much more pronounced. The effect of particle density on the collection efficiency has also been investigated. When particle gravity is included, the results show that √St50 is not affected by particle density ranging from 0.5 to 10 g/cm3, although the particle collection efficiency increases slightly with increasing particle density at high ends of the collection efficiency curves at high nozzle Reynolds number due to an ultra-Stokesian effect. The particle interception effect does not affect the collection efficiency curves at high Reynolds numbers at all, and the effect is negligibly small at low Reynolds numbers.