A new method is introduced for the measurement of percentage modulation for ultrasonic light intensity modulator by means of low frequency modulation. Difficulties arising from conventional photo-electric mesurement systems in the simultaneous measurement of D. C. and H. F. alternative components may be overcome. D. C. components of the photo-electric signals and a low frequency signal whose amplitude equals to 1/2 of the HF alternative components are displayed on an oscilloscope so that the rigional definition of percentage modulating can be used to calculate its percentage modulation.