Abstract: By means of RDS detection system, slight difference of reflectance coefficients for samples can be measured in two mutually vertical directions on the plane of the sample under approximately perpendicular incident condition, this is known as in plane optical anisotropy. It plays a significant role in the studies of the in plane optical anisotropy of the lower dimensional structures of semiconductor materials their quantum well superlattices, or the semiconductor surface restructuring, or the real time monitoring in the semiconductor epitaxy growth process. The present system has provided a great number of data for research work.