The classification performance of an improved rectangular jet virtual impactor having clean air entrances has been studied with particular attention to the effect of acceleration distance. Experimental results showed when the acceleration distance was shorter than a certain value, the classification performance became extremely worse. Flow visualization revealed this performance deterioration was attributed to the distortion in the flow field. Experimental results obtained under no flow distortion showed that longer acceleration distance caused a lower classification performance in the low inertia region because of particle diffusion, whereas shorter distance gave a lower performance in the high inertia region. However, it was found that the deterioration in the classification performance could be avoided if the partition plate for the wall side clean air entrance was set at shorter distance from the jet and that for the slit side clean air entrance was set at relatively longer distance.