Microstructure of TiN/Si₃N₄ ceramics was tailored by using a cosintering process of their agglomerates, then structure-properties relations were examined by measuring mechanical property and electrical conductivity of TiN/Si₃N₄. The TiN/Si₃N₄ material using TiN and Si₃N₄ agglomerates showed duplex microstructure. This materials showed a sudden onset of electrical conduction at a lower TiN content than that of conventional composites. The duplex materials exhibited a higher fracture toughness than that of the normal composites, while other hand its strength remained at the same level of TiN. It is concluded that higher fracture toughness of duplex materials resulted from the complex fracture path at the interface between Si₃N₄ and TiN agglomerates.