Abstract: SiC-diamond polycrystalline materials doping by diamond with different mass fraction were prepared by pressureless sintering with AlN−Y₂O₃−Sc₂O₃ as liquid phase. The microstructure and thermal properties were analyzed by scanning electron microscopy and laser flash method, respectively. The effects of mass fraction (1.0%, 2.5%, 5.0%) and particle size (0.25 μm, 1.00 μm) of diamond on the densification and mechanical properties of the SiC-diamond polycrystalline materials were studied. The results show that, the relative density of the sintered samples is over 94% when the diamond mass fraction is below to 5.0%, while the relative density of the sample with 5.0% diamond is lower than that of other samples. The relative density of SiC polycrystalline materials decreases with the increase of diamond content, and the excessive diamond in raw materials may reduce the densification of samples. Under the experimental conditions, the grain size does not grow abnormally. The hardness, fracture toughness, and bending strength of the samples are in ranges of 16~18 GPa, 3.8~4.4 MPa·m^1/2, and 239~540 MPa, respectively. The thermal conductivity and thermal diffusion coefficient of samples decrease with the increase of temperature, and the porosity is the main factor affecting the thermal conductivity of the sintered samples.