Virtual impactors can be used as aerosol concentrators as well as particle size classifiers. The flow field and particle trajectory inside a linear slit-type virtual impactor were investigated using a commercial computational fluid dynamics software package. Effects of flow and slit geometry, including the inlet flow (Q(in)), the ratio of minor to total flow (r), and the configuration of the taper-lip slit nozzle and the collection slit nozzle, were studied. A new modified Stokes number (Stk(c)) for the virtual impactor was proposed to predict the d(50) at different r values. Stk(c,50)(0.5) was found to be retained at approximately 0.9 under different Q(in) or r values, and it can be considered the characteristic performance parameter for the slit virtual impactor. The numerical simulation results show that the particle loss starts increasing when Stk(c)(0.5) is larger than 2.0 due to a particle-crossing phenomenon. The range of the maximum concentration factor (CFmax) is influenced by the particle-crossing phenomenon. An expression was also developed to predict the size range of particles with the maximum CF under different inlet flow rates. Regarding the geometrical configuration, the arc-lip nozzle could postpone particle-crossing and lessen the internal losses of coarse particles, whereas it increased particle losses near d(50). However, the divergent collection slit nozzle could effectively decrease particle losses on the wall of the nozzle without this side effect. The designing guidelines of a linear-slit virtual impactor are briefly summarized based on these findings.