Abstract: The slique is one of the most important organs to store the energy matter in an oilrape plant, a typical oil crop in China. It is necessary to measure the seed number per silique of oilrape for the breeding traits, even for the estimation of rape yields. But the current manual counting cannot fully meet the requirement of modern breeding, due to the time-consuming, labor-intensive, and less accuracy for the seed number per silique of oilrape, particularly by manually breaking the pod of rape. In this study, Image recognition of the silique length was proposed to count the seed number per silique of oilrape. Three varieties of oilrape were also used as the experiment materials. The siliques were first selected from the upper, middle and lower parts of rape plants. After that, a digital calliper was utilized to measure the length of silique pod without considering the fruit stalk and beak part. A correlation model was then established for the length of silique and seed number per silique. The images of siliques were also captured by a scanner in the tiled state. The image preprocessing included the gray scale, binarization, hole filling, and the removal of small areas. Meanwhile, the fruit stalk and fruit beak of silique were removed from the images of siliques using the opening operation of mathematical morphology. After that, the images were segmented to the sub-images of siliques with the single connected domain. Each sub-image of siliques was then thinned to a line with one pixel. The endpoint and intersection point of siliques images were finally obtained using hit-or-miss transformation. The Density Based Spatial Clustering of Applications with Noise (DBSCAN) was also used to remove the redundant intersection points. The endpoints of silique images were classified and paired to recognize the single silique. As such, the number of image pixels was obtained for the length of a single silique. The actual length of silique was calculated by a relative formula between the single pixel and actual length. Correspondingly, the seed number per silique was finally calculated from the images using the correlation model between the actual length of silique and seed number per silique. The determined coefficient for the correlation model between the length of silique and seed number per silique in the three varieties were 0.891, 0.881, 0.887, respectively, indicating a better performance of the estimation on the seed number per silique using the length of the silique. Furthermore, the average recognition accuracy was 97.25% for the length of silique, and the average prediction accuracy of seed number per silique was 83.87% using image processing. Consequently, image processing can be widely expected to realize the automatic recognition and counting of the seed number per silique for higher efficiency of rape production in modern agriculture.