Y₂O₃-SiO₂-MgO-incorporated silicon nitride (Si₃N₄) ceramics were investigated systemically to obtain cost-effective production and the effect of sintering temperature on the microstructure and thermal properties was studied. An α-Si₃N₄ starting powder was used for preparing the samples and sintered under a pressure of one bar nitrogen gas pressure at 1650, 1700, 1750, 1800, and 1850 ℃ for 2 h. The crystallisation behaviour of the ceramics was promoted by sintering temperatures as evidenced by the XRD results. A different secondary phase, melilite (Y₂Si₃O₃N₄) formation, was detected using SEM micrographs of the final microstructures of the samples sintered at higher temperatures above 1700 ℃ which had a positive effect on thermal properties. The values of thermal conductivity increased by about 75.5% with increasing sintering temperature. The highest thermal conductivity (53.58 W/(mK)) was achieved with the highest sintering temperature of 1850 ℃. The results revealed that ternary sintering additives, such as Y₂O₃-SiO₂-MgO, had a potential for low-cost production of Si₃N₄ ceramics with enhanced thermal properties.