The present study prepared molybdenum trioxide (MoO3), the most important intermediate of molybdenum metal, by using a fluidized bed reactor for the thermal decomposition of ammonium molybdate (AM) in the presence of an air flow. During the process of fluidizing the sample inside the reactor, the reaction time and temperature were optimized with a close analysis of the X-ray diffraction (XRD) data and with thermogravimetric analysis (TGA). In particular, the temperature level, at which the AM decomposition is completed, is very important as a primary operating parameter. The analysis of the XRD and TGA data showed that the AM decomposition is almost completed at ~350 ℃ with a reaction time of 30 min. A shorter reaction time of 10 min. required a higher reaction temperature of ~500 ℃ with the same air flow rate to complete the AM decomposition. A sharp rise in the decomposition efficiency at a temperature ranging between 320 and 350 ℃ indicated a threshold for the AM decomposition. The operating conditions determined in this study can be used for future scale-ups of the process.