Abstract: An automatic counting method of steel bar in a bundle based on connected domain area of image was proposed. The steel bar images are first segmented after gray processing and filtering. Then connected domains in segmentation images are classified into three groups: the domains with area smaller than basic area are considered as noise, the domains with area between basic area and threshold area are representative of one steel bar, and the others are multi-steel bar, whose area are divided by standard area to get amount of steel bar. The amount of all steel bars in the image is gotten through summing up amount of steel bars in each segmentation image. The experiments with 13 images showed that all the counting errors are smaller than 3% by use of the method.