A laser interferometer for sample stage positioning of electron beam lithography is developed. According to the characteristics of the electron beam lithography the optical arrangement and elements are specially considered. High resolution and stability are realized. The optical system of the interferometer, calculated results and the phase change method by polarizers are described. This interferometer setted in a vacuum chamber of a electron beam lithography system has been reliably working over one year. Eepeatabilitj of positioning is determined to be 0.04 micron.