The effects of beta-Si3N4 whisker additions on the mechanical properties of alpha’-SiAION ceramics were studied. The room temperature fracture toughness and fracture strength of the composites increased with increasing whisker content, and were 6.5 MPa m(1/2) and 900 MPa, respectively, for the addition of 30 vol% whiskers. Although creep resistance of the composites was not enhanced at 1200 degrees C, the whisker additions were observed to be beneficial in reducing the oxidation induced slow crack growth of alpha’-SiAION that occurred at 1300 degrees C, and thereby, improved the creep resistance of the composites at 1300 degrees C.