This paper will present sintered reaction-bonded silicon nitride (SRBSN) material with a high thermal conductivity of 121 W.(m.K)(-1), which has been successfully prepared from a coarse Si powder with lower levels of oxygen and aluminum impurities, using a mixture of Y2O3 and MgSiN2 as sintering additives, by nitriding at 1400 degrees C for 8 h and subsequent post-sintering at 1900 degrees C for 12 h at a nitrogen pressure of 1 MPa N-2. This thermal conductivity value is higher than that of the materials prepared from high-purity alpha-Si3N4 powder (UBE SN-E10) with the same additive composition under the same sintering conditions. In order to study the effects of Si powder characteristics on the processing, microstructure, and thermal conductivity of SRBSN, the other type of fine powder with higher native oxygen and metallic impurity (typically Al and Fe) contents was also used. The effects of Si particle size, native oxygen, and metallic impurities on the nitriding process, post-sintering process, and thermal conductivity of the resultant SRBSN materials were discussed in detail. This work demonstrates that the improvement in thermal conductivity of SRBSN could be achieved by using higher purity coarse Si powder with lower levels of oxygen and aluminum impurities. In addition, this work also shows that the nitriding temperature has no significant effect on the microstructure and thermal conductivity of SRBSN during post-sintering, although it does affect the characteristics of RBSN formed during nitridation.