The gelcasting and reaction sintered Si₃N₄ based ceramics is applied to critical component in nonferrous metal smelting and casting industries due to its excellent properties，However，it still had disadvantages of gas tight，antioxidation and toughness.In this study，Si powders and clay are used as raw materials to prepare gelcasting and reaction sintered Si₃N₄-O''-Sialon multi-phase ceramics.The results show that the clay can improve the processing properties of slurry，green body and sintering.The closed pores sealing effect caused by the high temperature liquid phase of clay and the sintering densification effect can significantly reduce the apparent porosity and increase the bending strength.Dissolution and nitridation of high temperature liquid phase can promote crystallization of β-Si₃N₄ and O''-Sialon from liquid phase.With the nitridation temperature increasing，the morphologies of O''-Sialon change from amorphous and granules into matured crystalline of needle-like，columnar-like and polyhedron-like grains with larger scale，leading to a toughening effect.The appropriate range of Si content and nitridation temperature are 70% to 80% and 1 500 to 1 600 ℃，respectively.For the sample of 80% Si and 1 600 ℃，the apparent porosity，bulk density，bending strength，work of fracture and critical stress intensity factor are 14.15%，2.22 g/cm³，114.0 MPa，450.3 J/m² and 1.88 MPa·m^1/2，respectively.