A novel composite materials using silicon nitride (Si₃N₄) as the substrate and diamond particles as the reinforcement phase were developed to increase both thermal conductivity and mechanical properties. The Ti coating on the surfaces of the diamond particles facilitated the formation of a titanium carbonitride (TiC_iN_1-i) interface between the two constituents during sintering, creating a strong bonding for high thermal conduction at the diamond-Si₃N₄ interface and inhibiting the graphitization of diamond during the sintering process. Furthermore, a sandwiched material design was made whereby Si₃N₄ and Si₃N₄/Ti-coated diamond layers were stacked alternately to endow the composites with a directional heat conduction characteristic. The thermal conductivity of the fabricated Si₃N₄/diamond composites increased by up to 272.87 % compared to that of commercially available Si₃N₄, making them excellent candidates for thermal management materials required in high-performance electronic devices.