The effects of the magnesium compound and yttria additives on the processing, microstructure, and thermal conductivity of sintered reaction-bonded silicon (Si) nitride (SRBSN) were investigated using two additive compositions of Y2O3-MgO and Y2O3-MgSiN2, and a high-purity coarse Si powder as the starting powder. The replacement of MgO by MgSiN2 leads to the different characteristics in RBSN after complete nitridation at 1400 degrees C for 8 h, such as a higher beta-Si3N4 content but finer beta-Si3N4 grains with a rod-like shape, different crystalline secondary phases, lower nitrided density, and coarser porous structure. The densification, alpha ->beta phase transformation, crystalline secondary phase, and microstructure during the post-sintering were investigated in detail. For both cases, the similar microstructure observed suggests that the beta-Si3N4 nuclei in RBSN may play a dominant role in the microstructural evolution of SRBSN rather than the intergranular glassy chemistry during post-sintering. It is found that the SRBSN materials exhibit an increase in the thermal conductivity from similar to 110 to similar to 133 (Wm.K)(-1) for both cases with the increased time from 6 to 24 h at 1900 degrees C, but there is almost no difference in the thermal conductivity between them, which can be explained by the similar microstructure. The present investigation reveals that as second additives, the MgO is as effective as the MgSiN2 for enhancing the thermal conductivity of SRBSN.