The explosion occurrence rate of metallic powder in Taiwan's nanometer grinding factory is 100%. The stage of grinding metal to nanometer is a stage of high risk. This study investigates the impact of titanium and iron particles using air at initial speed of 13.1m/s to pneumatic valve. Titanium powder of 30nm and iron powder of 15nm all result in explosion phenomena, whereas titanium powder of 45μm and iron powder of 150μm do not have such phenomena. The minimum ignition energy (MIE) of the nanometer class titanium and iron are all less than 1mJ, whereas the MIE of 45μm titanium powder is 21.91mJ and the MIE of 150μm iron powder is greater than 1000mJ.