Chemical reaction under microwave field goes noticed as the heating mechanism has been studied. It was reported that a reduction caused by microwave heating shows different scientific behavior from that by conventional ones. Authors noticed that metal titanium powders are heated by microwave heating at high oxygen partial pressure and the oxygen partial pressure change of exhaust-gas were measured to understand the mechanism of the nitriding response using solid electrolyte oxygen sensor. The nitridation mechanism was discussed using quantum-chemical simulation for Ti14O13 model. During application of the heating, the pressure decreased to 1 × 10 -10 atm and quickly increased to 1 × 10-5 atm and then oxygen emission peaks were observed. Furthermore, quantum-chemical simulation showed that Ti-O bonding of titanium oxide is anti-bonding and consists of 3d orbital.