SiCp/Sialon ceramic manix composites were fabricated by pressureless sintering method using the Sialon powder synthesized from the kaolin clay and submicron SiC particles. The best sintering parameter of SiCp/Sialon composites with 4% (mass fraction) Y2O3 is 1500 ℃×2 h. It is shown that with the increasing of SiCp content, the bulk density decreases, the apparent porosity increases, the flexure strength increases at first and then decreases, the fracture toughness increases and hardness gradually decreases. The best physical properties can be obtained with 10 mass% SiCp, and the bulk density is 3.06 g ·cm-3, apparent porosity is 2.4％, flexure strength is 389.5 MPa, and Vickers hardness is 18.4 GPa. There will be 1280 MPa radial tensile stress and 640 MPa tangential stress in side of Sialon of interphase boundary between SiCp and Sialon phase by calcilation. The mechanism of improvement of SiC content on mechanical properties are also discussed.