A method with high precision for measurement of optical retardation angles of wave plates is presented, based on laser birefringent longitudinal-mode-splitting technology. When a wave plate is placed in the laser cavity, a laser longitudinal mode will be split into two modes because of the birefringence of the wave plate. The optical retardation angle of the wave plate is proportional to the ratio of the frequency difference between the two modes to the laser longitudinal-mode interval. The retardation angle can be determined by measuring these two frequencies. The method uses a compact setup and attains high resolution and accuracy. The error is less than 8 arcmin. The paper discusses a crystal quarter-wave plate at 632.8-nm wavelength as an example, but the method is suited for other laser wavelengths and wave plates. © 2001 Society of Photo-Optical Instrumentation Engineers.