In the present study, we report the effects of starting beta-Si3N4 particle sizes and post-sintering heat treatment on microstructure evolution and mechanical properties of prepared alpha-beta SiAlON ceramics. Three different beta-Si3N4 starting powders, with particle sizes of 2, 1 and 0.5 mu m were used to prepare alpha-beta SiAlON ceramics by gas-pressure sintering. Elongated beta-SiAlON grain morphology was identified in the samples prepared using 0.5 mu m particle size beta-Si3N4 powder. Low-aspect ratio grain morphology was observed in samples prepared from starting powders with coarse particles (2 mu m and 1 mu m). The sintered samples were further heat treated to develop desired microstructure with elongated grains. The hardness and indentation fracture toughness values of sintered and heat treated samples were found to lie in the range of 12.4-14.2 GPa and 5.1-6.4 MPa m(1/2) respectively. It was revealed that fracture toughness increases with decrease in particle size of starting beta-Si3N4 powder. (C) 2010 Elsevier Ltd. All rights reserved.