The number of aged power equipment increases and it is important to clarify the characteristics of deteriorated insulating oil. This paper describes the basic study on transmittance spectra of oxidized insulating oils by terahertz spectroscopy. First, the mineral and mixture insulating oils heated in oxygen were measured and observed spectra change. Second, the oxidation process of insulating oil was modeled by using various compositions of dodecane and 2-octanol. The absorption related to hydroxyl groups (-OH) was significantly observed between 6 and 7 THz. And hydrogen bonds were observed from vibration change of OH. The absorption band was also observed in accelerated aging insulating oils used in power equipments as insulating oil.