Electron Spin Resonance (ESR) spectrometry is an analytical method to observe and measure absorbed energy on irradiation of the microwave to the radicals in a strong magnetic field. Recently, the function of active oxygen species, such as the hydroxyl radical and super oxide in vivo, has attracted attention. These radicals can be studied by the ESR spectrometry. Generally, these radicals are unstable, but when they are first reacted with a spin trapping reagent, they can be trapped and stabilized. On the other hand, with use of a spin labeling reagent, the molecules without radicals can also be measured with ESR.