Nanosized and highly reactive magnesium niobate Mg4Nb2O9 were successfully synthesized by a sol-gel method as potential candidates for use in microwave applications as a low-temperature co-fired ceramic (LTCC). The Nb-Mg-citric acid solution was evaporated resulting in a dry gel that was calcined in the temperature ranging from 550°C to 850°C for crystallization in air atmosphere. The crystallization process from the gel to the crystalline Mg4Nb2O9 was followed by thermal analysis and during calcinations phase evolution of the powders was studied using X-ray diffraction (XRD) technique. Particle size and morphology were examined by high resolution scanning electron microscopy (HR-SEM). At 850°C the corundum-like phase of magnesium niobate was obtained in air atmosphere with an average particle diameter of 80 nm approximately calculated from the HR-SEM images.