Abstract: A new accurate method for measuring the birefringence of muscovite mica has been reported in terms of the dispersion relations of the indices and the variety relations between the phase delay and the wavelength.Through measuring the thickness of mica plates and the wavelengths at which they act as quarter wave phase retarders,the birefringence can be obtained in terms of the function relation between the phase delay,the wavelength and the thickness.This method is simple and the accuracy of the measurement is in the order of 10-5.