Si3N4 powders with the concurrent addition of Yb2O3 and MgSiN2 were sintered at 1900 degreesC for 2-48 h under 0.9 MPa nitrogen pressure. Microstructure, lattice oxygen content, and thermal conductivity of the sintered specimens were evaluated and compared with Si3N4, Yb2O3, and MgO addition. MgSiN2 addition was effective for improving the thermal conductivity of Si3N4 ceramics, and a material with high thermal conductivity over 140 W-.(m(.)K)(-1) could be obtained. For both specimens, lattice oxygen content was decreased with sintering time. However, the thermal conductivity of the MgSiN2-doped specimen was slightly higher than the MgO-doped specimen with the same oxygen content.