Abstract: Typical O'-sialon-based ceramics, with a formula of Si_2-xAl_xO_1+xN_2-x, where x was set as 0.25, were fabricated by in-situ synthesis. Si₃N₄, Al₂O₃, and SiO₂ powders were used as raw materials, and MgO and Y₂O₃ were added as sintering additives. All the samples were sintered at different temperatures under a nitrogen pressure of 0.25-0.30 MPa, and their microstructure, phase content, and thermal conductivity were evaluated. The effects of O'-sialon and β-Si₃N₄ on the thermal conductivity were analyzed by numerical calculation in detail. In the case of the similar porosity, the thermal conductivity of O'-sialon-based ceramics decreased with the ratio of O'-sialon/β-Si₃N₄ increasing. When the ratio was 12, the thermal conductivity of O'-sialon ceramics sintered at 1360℃ was 1.197 W·m^-1·K^-1.