The marker for quantitative determination of free radicals by a radio frequency (RF) electron spin resonance (ESR) spectrometer operating at about 700 MHz was found for the first time. It is zinc(II) oxide (ZnO) with the ESR signal due to oxygen vacancy. The g-value was about 1.96 and also the line shape was sharp. When this marker was measured together with TEMPOL aqueous solution by using the RF ESR spectrometer, each signal did not overlap. Furthermore, the signal intensity ratio of TEMPOL to ZnO unchanged even if Q-value of a resonator was decreased.