Mullite (3Al₂O₃·2SiO₂) has emerged as a promising candidate for high-temperature structural materials due to its erosion resistance, chemical and thermal stabilities, relatively low thermal expansion coefficient, excellent thermal shock and creep resistances, and low dielectric constant. However, since the pure mullite sintering temperature is as high as 1,600~1,700℃, there is an increasing need for a sintering additive capable of improving the strength characteristics while lowering the sintering temperature. Herein we have tried to obtain the optimal sintering additive composition by adding MgO, Cr₂O₃, and Y₂O₃ to mullite, followed by sintering at 1,325~1,550℃ for 2 h. With additives of 2 wt% of MgO, 2 wt% of Cr₂O₃, 4 wt% of Y₂O₃, A density of 3.23 g/cm³ was obtained for the sintered body at 1,350℃ upon using 2 wt% MgO, 2 wt% Cr₂O₃, and 4 wt% Y₂O₃ as additives. The three-point flexural strength of that was 275 MPa and the coefficient of thermal expansion (CTE) was 4.15 ppm/℃.