In this paper an optical profilometer based on the principle of heterodyne interferometry has been developed. A new optical path design with a two-parallel-path is presented. This instrument will be insensitive to the vibration the air turbulence and no high precision reference surface is needed. The measurement results show that the vertical resolution is better than mm and the lateral resolution is less than 1 I. L m. Based on this design we also analyze its potential errors such as the error caused by the distance between two parallel optical paths that caused by the phase detection system and those casued by the optical design and others. The stability and the repeatability of the instrument are also evaluated.