The onion powder with an initial moisture content of 10% (dry basis) in a thin infinite layer of thickness 5 mm was dried using a laboratory microwave oven at 50 W/g microwave power density to a final moisture content of 2% (dry basis). The temperature and moisture change pattern of the experimental process was monitored. Computer simulation of the predicted temperature pattern with drying time was obtained by using the mathematical model governed by the energy balance equation and using data of the thermo-physical properties of the onion powder. The code for the simulation was written using the C++ programming language. The high microwave power-to-mass ratio resulted in raised temperature with reduced drying time. The initial low moisture content of the product led to a gradual decrease in the drying rate. However, effective moisture diffusivity showed an increasing trend with decreasing moisture content because of the application of high-power density. Lethality values for Bacillus cereus spores, which were more prominent in onion powder, were evaluated from the simulated temperature curves. It was observed that sterilization of B. cereus started after 75 s at 117 °C with 0.431 (lg (CFU/g)) reduction. The maximum temperature of 117 °C was limited due to the unacceptable sensory quality of the product.