A variety of combinations of Y2O3 and MgO were used as additives in preparing Si3N4 ceramics by the sintering of reaction-bonded silicon nitride (SRBSN) method. By varying the amount of Y2O3 in the range of 0-5 mol% and that of MgO in the range of 0-8 mol%, the effects of Y2O3 and MgO additives on nitridation and sintering behaviors as well as thermal conductivity were studied. It was found that appropriate amount and combination of Y2O3 and MgO additives were essential for attaining full densification and achieving high thermal conductivity. The sample doped with 2.5 mol% of Y2O3 and 5 mol% of MgO attained a thermal conductivity of 128 Wm(-1)K(-1) when sintered at 1900 degrees C for 6 hours, and the sample doped with 2 mol% of Y2O3 and 4 mol% of MgO achieved a thermal conductivity of 156 Wm(-1)K(-1) when sintered for 24 hours.