In this work, an apparatus is developed to measure the oxidation of electrical insulating oil. The apparatus can be mounted onto mechanical devices for detecting the intensity of fluorescence light reflected from the oil in real time, and this detected intensity is an indication of the extent of oil oxidation. The developed apparatus evaluates the degree of oil oxidation of test oils, based on the UV fluorescence emission ratio. The test results obtained herein are compared with those obtained from other measurement methods such as the titration method and IR spectroscopy.