A novel ZrSi2-MgO system was used as sintering additive for fabricating high thermal conductivity silicon nitride ceramics by gas pressure sintering at 1900 degrees C for 12 hours. By keeping the total amount of additives at 7 mol% and adjusting the amount of ZrSi2 in the range of 0-7 mol%, the effect of ZrSi2 addition on sintering behaviors and thermal conductivity of silicon nitride were investigated. It was found that binary additives ZrSi2-MgO were effective for the densification of Si3N4 ceramics. XRD observations demonstrated that ZrSi2 reacted with native silica on the Si3N4 surface to generate ZrO2 and beta-Si3N4 grains. TEM and in situ dilatometry confirmed that the as formed ZrO2 collaborated with MgO and Si3N4 to form Si-Zr-Mg-O-N liquid phase promoting the densification of Si3N4. Abnormal grain growth was promoted by in situ generated beta-Si3N4 grains. Consequently, compared to ZrO2-doped materials, the addition of ZrSi2 led to enlarged grains, extremely thin grain boundary film and high contiguity of Si3N4-Si3N4 grains. Ultimately, the thermal conductivity increased by 34.6% from 84.58 to 113.91 W center dot(m center dot K)(-1) when ZrO2 was substituted by ZrSi2.