The effects of inlet and outlet dimensions on the performance of virtual cyclones were investigated in this study. Particle collection efficiencies and pressure drops for four virtual cyclones with different inlet/outlet widths were measured at various flow rates ranging from 10 to 50 L min(-1). The results show that the virtual cyclone with a narrow inlet/outlet results in improved collection efficiency and relatively high pressure drop. However, with the cost of the same pressure drop, the narrowest inlet/outlet achieved the smallest cutoff diameter. In addition, increasing flow rate enhances the efficiency and increases the sharpness of the efficiency curves. Comparison between experimental data and estimation of Torczynski and Rader's theory reveals that the theory failed to predict the performance of virtual cyclones, suggesting the necessity of more elaborated work to understand the separation mechanisms of virtual cyclone.