Machinable BN/Si3N4 and electroconductive TiN/Si3N4 nanocomposites were prepared, using powders synthesized through an in-situ nitridation method in flowing ammonia gas. Due to the homogeneous mixing of various components in the powders, nanocomposites with homogeneous second phase distribution in the matrix were obtained. These nanocomposites showed enhanced strengths and distinctive functionalities. BN/Si3N4 nanocomposite with 20-25vol% BN showed a relatively high strength of over 700 MPa and was able to be machined into complicated shapes with diamond bits. Electroconductive TiN/Si3N4 nanocomposite with 25vol% TiN showed a high strength of 1100MPa and low electrical resistivity of 1.1×10-2 ⋅cm, and was promising for electrical discharge machining.