Abstract: [Objective] To design a kind of software which can grade the size of walnut online by machine vision.[Method]In this paper,the Daheng USB2.0 Industrial Camera was used to obtain the RGB image of walnut on-line detection,and the interface of program was developed by VS2010 to convert the image data and preprocess,which could obtain the contour of image.By using OpenCV image moment calculation principle to extract walnut dimensions data,the software could provide different levels of signal for the lower computers to control solenoid valve action.[Result]The software displayed the images before and after the processing and realized the optimization of image acquisition and the manual adjustment of segmentation threshold.[Conclusion]The walnut size software based on machine vision can detect walnut with different sizes and send out grading signal accurately.