Al2O3-B4C whisker composites have been successfully densified by pressureless sintering. Low oxygen partial pressures, below 10(-10) atmospheres, were determined to be suitable for the composite densification, and the maximum densities were achieved at 1800 degrees C for 60 minutes. The B4C whiskers were screened classified into five size fractions and the effect of size and size distribution on the densification were studied. The small whiskers with a narrow size distribution (-325 + 400 mesh) yielded the highest density results. The maximum value of relative densities was 98%, 97%, 94% and 89% at 10, 20, 30 and 40 vol % B4C whiskers, respectively. Addition of B4C between 5 and 15 vol % proved to be a sintering aid to the alumina densification via mechanisms other than being a grain growth inhibitor. An increased fracture toughness up to 6.2 MPa.m(1/2) was achieved in the composites containing 10-20 vol % B4C whiskers.