A machine vision system for bearing outer diameter inspection has been developed in this paper. The infinite objective of optical system is applied to diameter measurement system. The paper provides a method for measuring the size of objects based on polynomial interpolation. The technology of CCD image stagger is applied in the paper. The method can avoid space between pixels astriction theoretically, so higher measurement accuracy is gained. The experiment results show that the system has many advantages such as non- touch , high speed and higher precision.