In 2005, an investigation conducted on three nanoscale attrition millers in Taiwan revealed that all three had undergone metal nanopowder explosions in the past. This research was aimed at determining the relationships between the particle diameter of an aluminum nanopowder and its maximum explosion pressure (Pmax), maximum rate of pressure rise ((dP/dt)max), minimum explosion concentration (MEC), and minimum ignition energy (MIE) by 20 liter apparatus and 1.2-L Hartmann apparatus. The results revealed that 35-nm aluminum powder has a Pmax of 7.3 bar and deflagaration index (KSt) of 349 bar•m/s, in 100-nm aluminum powder, Pmax of 12.5 bar and KSt of 296 bar•m/s and 40-μm aluminum powder, Pmax of 5.9 bar and KSt of 77 bar•m/s. The value of (dP/dt)max for the 35-nm aluminum powder is 4.5 times that for the 40-μm aluminum powder. The 35-nm, 100-nm, and 40-μm powders have MEC values of 40, 50, and 35 g/m3, respectively. The 35-nm and 100-nm powders both have MIEs less than 1 mJ, while the 40-μm powder has an MIE of 59.7 mJ.