Burn rates were experimentally determined for nanocomposite thermite powders composed of aluminum (Al) fuel and molybdenum tri-oxide (MoO3) oxidizer under well-confined conditions. Confined pressures were also measured. Samples of three different fuel particle sizes (44, 80, and 121 nm) were analyzed to determine the influence of particle size on burn rate. Bulk powder density was varied from approximately 5 to 10 percent. The burn rates ranged from approximately 600 to 1000 m/s. Results indicate that burn rate increased with decreasing particle size, and the smallest particle composite also exhibited less variation in burn rate as the bulk density increased.