A technique for forming a rotator by retarders is presented. The principle to form the rotator and its application are analyzed with Mueller matrix. When a retarder is placed between two quarter-wave plates whose fast axes are perpendicular to each other and the angles between fast axes of the retarder and the two quarter-wave plates are ±45° respectively, the rotator is formed. Based on the technique, a phase modulator can be applied for the modulation of polarization direction. In the experiments, principle of forming the rotator and application of the phase modulator to modulation of polarization direction are verified.