Discharge characteristics of polydisperse powders through a conical hopper were experimentally investigated by observing the solids discharge regimes under different positive pressure differences (i.e., P0) and also measuring the corresponding solids discharge rates. The observations and measurements were systematically conducted at least four diameters of hopper orifice (2.5mmDo30.0mm) under five positive P values (0PaP1405Pa) for the test powders: river sand, zeolite, fertilizer, and sawdust. Effects of positive P on both discharge regime and solids discharge rate are first predicted for different orifice diameters and test powders in this study. One of the novel results on the discharge regime is that increase of positive P value up to a critical value conserves or successively stabilizes the discharge regime observed when P=0 (i.e., for gravity flow condition) through the same hopper. However, the additional increase of positive P over this critical value destabilizes the discharge regime, as in conformity with the effect of increase in negative P cited in the literature.