First-order linear ﬁlter is a wide application algorithm to detect edge in digital image. However it dosen’t make good effort to the image where contrast varies much, or luminance takes on non-uniform. In this paper, a fuzzy inference system (FIS) is made up and used to detect edges. The experiment shows that FIS is much better in edge detection when the image with high contrast variation than with the linear Sobel operator. The FIS is also more precise in edge detection than Sobel operator.