Abstract: Automatic diamond cut measuring system is an accurate and rapid measuring system combined with the digital image processing.Some key corners of diamond projection image are detected by using some image processing methods.The diamond cut ratios,such as girdle diameter,total depth,pavilion depth,pavilion angle,crown depth,crown angle,table size,girdle depth and the other related symmetry parameters,can be calculated with those key corners.The hardware and software of the automatic diamond cut measuring system developed by Gemmological Institute,China University of Geosciences(Wuhan),are mainly described in the paper.