Molybdenum trioxide is a well known heterogeneous catalyst and is used in many additional applications from secondary lithium batteries to large area display devices. Chemical sensors have also been demonstrated using MoO₃ to detect ammonia. Molybdenum trioxide particles were synthesized using the flame spray pyrolysis (FSP) method. The nanopowders were characterized with X-ray diffraction (XRD), nitrogen adsorption specific surface area (BET), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and Raman spectroscopy. The produced particles crystallized with a plate-like morphology and consisted of α- or β-MoO₃. A flame condition was found, where a further crystallization and crystal growth took place on a post-flame filtering surface and resulted in very large, plate-like and uniformly crystallized α-MoO₃ particles. The thermal annealing and phase transformation leading to these structures was confirmed by a post-synthesis thermal treatment of as-produced powders at 500°C for 4 hours.