Spark plasma sintering (SPS) is a method to consolidate fine powders to produce ceramic parts. Due to thermal cycles and pressures applied during SPS, phase transformations occur in raw materials used for producing high performance ceramics. In this study alumina in two forms, a-and γ-Al2O3, and silica (SiO2) mixed with zirconia (ZrO2) were sintered to form zirconia-mullite (MMA and MMG, respectively). The properties of these ceramics were compared with those produced by sintering pre-synthesized mullite nano-powder with and without zirconia addition. The content of mullite formed was studied for specimens made with alumina and silica feedstocks. The raw materials were sintered at different temperatures to obtain the correct temperature for obtaining highest mullite content and high density. Phase composition, morphology and mechanical properties of the ceramics were studied. It was found that the mullite prepared at 1500 °C with γ-Al2O3 has a higher percentage of mullite phase. However, the specimen made with pre-synthesized mullite powder had the highest mechanical properties.