The effect of particle size and shape on the sintering response of aluminum powder has been examined. Spherical 3-, 5-, and 15-μm powders and irregularly shaped 6-, 7-, and 15-μm powders from two manufacturers were mixed with 4 wt pct Sn, poured into a crucible, and sintered for 2 hours under argon at 620 °C. The particle shape appears to be a critical characteristic governing the sintering characteristics. The particle size and size distribution, the tap density, the oxide film thickness, the surface chemistry, and the impurity concentration had little influence. The irregular particles sintered to a final density of 88 to 91 pct, whereas the spherical particles reached a density of only 65 to 73 pct. It is suggested that the differential thermal expansion between the aluminum particle and its oxide film may cause the oxide to fracture and that the fracture characteristics are different between the two powder morphologies.