A measurement system utilizing laser frequency splitting is presented for the precise measurement of retardations of arbitrary wave plates. The wave plate to be measured is inserted into a laser resonance cavity, and its retardation can be determined by measuring the difference of split frequencies and longitudinal mode spacing of the laser. In the system, mode spacing measurement is performed under multimode oscillation according to the relation between polarization directions of modes. To solve the problem of strong mode competition in measuring half-wave plates, a transverse magnetic field is applied to weaken the competition so as to make the system applicable even when the frequency difference is less than the threshold for strong mode competition. The error of the system is less than 5 arcmin, and standard deviations of about 1 arcmin are obtained from repeated experiments samples of quarter- and half-wave plates. No element rotation is required during measurement, and no mechanical quantity, such as angle, needs to be measured.