A particle trap impactor with a cup shaped collector was evaluated with varying geometrical and operating parameters. The acceleration nozzles having different diameters were designed, and the distance between the acceleration nozzle and the particle trap was changed. The aim of this work was to find out the accurate design parameters of particle trap impactors. Interestingly, our results indicated that root Stk(50) (i.e., root Stk of a particle having a 50% probability of separation) of particle trap impactors were about 0.6, whereas those of inertial impactors and virtual impactors were 0.49 and 0.69, respectively. It is apparent that root Stk(50) values of particle trap impactors can be set at 0.6, even though their design conditions are somewhat different. In addition, the penetration efficiency was found to be dependent on the ratio of nozzle-to-trap distance to nozzle diameter, the sampling flow rate and the nozzle diameter. (C) 2005 Elsevier Ltd. All rights reserved.