Research on digital image processing technology, which began in 1960s, stepped into an active research stage in late 1970s and early 1980s. It was firstly used in industrial and biomedical fields. Although it put into use in agricultural research very late, it has a broad prospect. In order to realize navel orange grading, this paper used Visual C # .NET program to develop navel orange shape and diameter rapid grading based on machine vision image feature. Southern Jiangxi navel orange was used as the research object. The color and shape feature of the navel orange was extracted. The image data was processed through Sobel Operator algorithm and standard median filter. Results show that the digital image processing technology based on C # program is feasible for shape grading of navel orange. It also provides a new method for navel orange grading detection.