An optical circular polarization modulator(CPM) based on liquid crystal(LC) is presented.Using the electric birefringence of LC material,the device can output polarized light with two circular-polarization states.The detailed illuminations of the device's structure and its testing investigation are also presented.This kind of CPM has the advantage of low drive voltage that is less than 10 V,fast response speed and small volume.The test results demonstrate the circular polarization degree of output light of this CPM is above 98.8 %.