The yttrium-sialon ceramics with the composition of Y0.333Si10Al2ON15 and an excess addition of Y2O3 (2 or 5 wt%) were fabricated by hot isostatic press (HIP) sintering at 1800degreesC for 1 h. The resulting materials were subsequently heat-treated in the temperature range 1300-1900degreesC to investigate its effect on the alpha-->beta-sialon phase transformation, the morphology of alpha-sialon grains, and mechanical properties. The results show that alpha-sialons stabilized by yttrium have high thermal stability. An adjustment of the alpha-sialon phase composition is the dominating reaction in the investigated Y-alpha-sialon ceramics during low-temperature annealing. Incorporation of excess Y2O3 could effectively promote the formation of elongated alpha-sialon grains during post-heat-treating at relatively higher temperature (1700degrees and 1900degreesC) and hence resulted in a high fracture toughness (K-IC = 6.3 MPa.m(1/2)) via grain debonding and pullout effects. Although the addition of 5 wt% Y2O3 could promote the growth of elongated a grains with a higher aspect ratio, the higher liquid-phase content increased the interfacial bonding strength and therefore hindered interface debonding and crack deflection. The heat treatment at 1500degreesC significantly changed the morphology of alpha-sialon grains from elongated to equiaxed and hence decreased its toughness.