Composite densification was studied by performing slip casting and sintering experiments on an Al2O3matrix and Si3N4whisker system. Even though all the slip‐cast powder compacts exhibited high green densities (up to 70% of the theoretical) and narrow pore‐size distribution (pore radius around 15 to 30 nm), significant differential densification on a microscopic scale was found due to the existence of local whisker agglomeration. The inhomogeneous whisker distribution resulted in a binary mixture of large and small pores in the sintered composites, in which whisker‐associated flaws remained stable even after prolonged sintering. The sintered microstructures showed that the spatial distribution as well as the volume fraction of the Si3N4affect composite densification. Inhomogeneous whisker distribution dominated the complete densification of the compo