The quarter-wave plate is the key device in the technology of polarized light. So it is important to determinate the retardation of the wave plate. A phase-modulated ellipsometry is described. By modulating the state of polatrzation, intensity signal changing with time is generated from the detector. The delay of the wave plate is obtained by analysing the harmonic component in the signal. The delay of four wave plates is measured, that of three wave plates among them has been precisly measured by electrooptic modulation method. By comparing the results of the two ways, it is found that they have little difference. The oscillation phenomenon in the delay of the isinglass wave plate is found,which had been proved in the reference 3. The experimental results show the phase-modulated ellipsometry is of significance.