Studies that replace a portion of the micron-size aluminum (Al) with nano-Al particles in an energetic formulation demonstrate significant performance enhancement. Little is known, however, about the critical level of nano-sized fuel particles needed to enhance the performance of the energetic composite. Ignition sensitivity and combustion velocity experiments were performed using a thermite composed of Al and molybdenum trioxide (MoO 3). Both loose powders and compressed pellets were examined. A bimodal Al particle size distribution was prepared using 4 or 20-μm-diam Al fuel particles that were replaced in 10% increments by 80-nm-diam Al particles until the fuel was 100% nano-Al. Results show that with only 20% nano-Al content, the mixtures showed reduced ignition delay times by up to 2 orders of magnitude. The combustion velocity was shown to dramatically increase as more nano-Al particles replace micron-Al particles within the mixture. This increasing trend was attributed to incomplete reactions of the micron-Al particles or significantly slower reactions such that the micron-Al particles promote cooling or quenching of the reaction.