Silicon nitride powders with concurrent addition of Yb₂O₃ and MgO were sintered at 1900°C for 2-48 h under 0.9 MPa nitrogen pressure. Microstructure, lattice oxygen content and thermal conductivity of the sintered specimens were evaluated. All specimens had a duplex microstructure composed of small matrix grains and elongated grains. Thermal conductivities of sintered bodies were remarkably increased with sintering time and reached a maximum value of about 120W·m^-1·K^-1. The lattice oxygen content of dense silicon nitride sintered body decreased with sintering time. The thermal conductivity increased with decreasing the lattice oxygen content. So far, it was reported that the thermal conductivity of Si₃N₄ was affected by grain size and morphology, composition and thickness of grain boundary layer. It was revealed that thermal conductivity of sintered specimen was closely related to the oxygen content present in the silicon nitride lattice.