Microwave sintered Si3N4-MgO system that contains 2, 4 and 10 wt% of ZrO2 as secondary particulates were investigated with respect to phase transformation and microstructure development. The experimental results of microwave sintered samples were compared with conventional methods. Complete α to β phase transformation was observed in the case of microwave sintered samples due to the volumetric nature of microwave heating. High temperature X-ray diffraction (HTXRD) analysis was performed to study in-situ the oxidation behavior of Si3N4 specimens. Si3N4 specimens with 10 wt % ZrO2 were exposed to air at temperature between 25°C and 900°C for up to 24 hours. Microwave sintered sample were structurally stable in air 25°C and 900°C for up to 24 hours of testing.