Amongst a variety of fabrication procedures for silicon nitride and related ceramics, reaction bonded processing has some advantages, such as low environmental impact, low sintering shrinkage and low raw material cost. In this study, silicon nitrides with high thermal conductivities over 80 Wm-1K-1 were prepared via nitridation of Si powder compacts followed by post-sintering (sintered reaction bonded Si3N4, SRBSN) . The thermal conductivities of SRBSN specimens were increased with increasing holding time. For example thermal conductivities of SRBSNs fabricated under reducing nitridation atmosphere followed by post-sintering of 3, 6, 12 and 24 hrs were 109, 125, 146, and 154 Wm-1K-1, respectively. Lattice oxygen contents were measured by the hot-gas extraction method. It was confirmed that thermal conductivity was increased with decreasing lattice oxygen regardless of additives, processing routes, indicating that the lattice oxygen is a decisive factor for fabricating high thermal conductivity Si3N4.