The virtual impactors are frequently used to concentrate particles, that are larger than the cut-off size, in a small volume of air, and thus to get a higher number concentration of larger sized particles. However, the conventional virtual impactors have disadvantage that particles that are smaller than the cut-off size can also be included in the concentrated aerosol. In this study, clean air was introduced into the virtual impactor in an effort to reduce the fraction of smaller particles in the concentrated aerosol. The collection efficiency of the virtual impactors were obtained both numerically and experimentally, with and without the clean air flow. It was found that the fraction of particles smaller than the cut-off size could be much reduced by introducing clean air into the virtual impactor (ysjnuri@hanyang.ac.kr).