Al-MoO3 nanocomposites were synthesized by arrested reactive milling (ARM) using powders of Al and MoO3 as starting materials. Varying milling parameters lead to different scales of structural refinement in the nanocomposites. The objective of this work was to determine the range in which the degree of structural refinement can be changed in a controlled manner. The effect of structural refinement on the characteristics of these reactive nanocomposites was further investigated. The milling intensity was controlled by using different milling media along with varying amounts of process control agent (PCA). X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Differential Scanning Calorimetry (DSC) and wire-ignition tests were performed to analyze the Al-MoO3 nanocomposites. Results indicate that the crystallite size decreased with increase in the milling intensity. However, increase in milling intensity also results in undesirable partial reaction between starting materials. Milling conditions resulting in the highest structural refinement and minimized partial reaction were identified. It was found that materials synthesized under the identified optimum conditions ignited at lowest temperature.