A calibrator of an OES may include a cover, a reference light source and a controller. The cover may be detachably combined with a ceiling of a plasma chamber of a plasma processing apparatus. The reference light source may be installed at the cover to irradiate a reference light to the OES through an inner space of the plasma chamber. The controller may compare a spectrum of the reference light inputted into the OES with a spectrum of an actual light inputted into the OES during a plasma process in the plasma chamber to calibrate the OES. Thus, the OES may be calibrated without disassembling of the OES from the plasma chamber to decrease a time for calibrating the OES.