In-situ synthesis and densification of hafnium diboride - silicon carbide (HfB2-SiC) ceramic composites were studied by the spark plasma sintering (SPS) method using HfO2, B, and SiC as starting powders. Influence of SiC particle size (similar to 2 mu m and similar to 10 mu m) on mechanical properties and oxidation resistance of in-situ composites were extensively investigated. According to the achieved results, the highest densification (similar to 95 % TD) was obtained with fine SiC particle reinforcement. The measured Vickers hardness and fracture toughness of the composites containing fine SiC were 14.3 GPa and 5.42 MPa.m(1/2), respectively. The fracture mode changed from transgranular cleavage to a mixed-mode, and the crack deflection was believed to be the primary toughening mechanism. From oxidation test result, the composite with fine SiC particles exhibited more inhomogeneous oxide scales compared to that of coarse SiC containing composite.