This thesis develops an Automatic Optical Inspection (AOI) system, which inspects defects of coaxial connectors. Inspected defects include missing, outside dimension and deformation of the coaxial connector. By adaptive thresholding and computing horizontal and vertical projections of images, the missing of coaxial connector can be inspected. Outside dimension and deformation are inspected by using image processing and improved Hough circle transform, which reduces the computation by projection information. Finally, the experimental results demonstrate the effectiveness of the proposed AOI system.