Calcium-stabilized alpha-sialon ceramics reinforced with cubic boron nitride (cBN) particles were synthesized using spark plasma sintering. The effects of the size of alpha-sialon precursor particle and the influence of the amount of cBN reinforcement on the physical, structural and mechanical characteristics of the composites were studied. Synthesis of the composites at 1500 °C with a holding time of 30 min resulted in densified composites. CBN-reinforced alpha-sialon composites synthesized via probe sonication showed particle sizes between 200 nm and 450 nm and showed significantly better mechanical properties than did the unreinforced sialon. A Vickers hardness (H_V10) value of as high as 24.0 GPa was measured for this reinforced sialon, as compared to a value of 21.6 GPa for the pure alpha sialon. However, a phase transformation from alpha to beta sialon was observed when using high energy ball milling process condition, where the size of the alpha-forming precursors was decreased to about 100 nm. This phase transformation was accompanied by a simultaneous cubic-to-hexagonal boron nitride phase transformation.