An adaptive modulation system for a liquid crystal (LC) phase modulator is demonstrated. The phase retardation may be modulated by resetting the driving voltage automatically by matching the measured and ideal transmittance of an LC cell sandwiched by crossed polarizers. By using this system, an LC phase modulator can get a low error function of 0.25% in a short modulation time, which is less than the 10% obtained using a conventional modulation method.