A crack detection method of concrete bridges was proposed by using digital image technology, and some image processing algorithms, including gray processing of image, calculation of pixel rate, image noise filter, and edge detection of crack, were analyzed and evaluated for the crack detection of concrete surface. Then, a computer program was developed with visual C++6.0 programming language to detect the cracks, which was tested with 15 cases of bridge video images. The results have indicated that this method can effectively detect the width of cracks for reinforced concrete bridges and the B type prestressed concrete bridges. The results have also showed that the relative error is within 6% for the more than 0.3 mm cracks and it is less than 10% for the crack width between 0.2 mm to 0.3 mm.