In this paper, molybdenum trioxide (MoO₃) nanoparticles were synthesized by rapid-microwave method using ammonium heptamolybdate (AHM) as a precursor in ethylene glycol (EG) solution with concentrated HNO₃. This reaction was carried out in a short period of 30 min and the nanoparticles were then heat treated at 600°C. The structures of the products were  analyzed by X-ray diffraction (XRD). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to record the morphology and nanoparticle size of MoO3. The Raman spectrum of MoO₃ displays three well-defined peaks located at 989.2, 816.0 and 665.3 cm^-1 which are the fingerprints of the orthorhombic α-MoO₃ crystalline phase.