Longissimus dorsi is the object of inspection for beef carcass grading based on computer vision, and marbling is a major index. Because locating the longissimus dorsi is an important requirement of a computer grading system, it is necessary to segment longissimus dorsi from ribeye imaging. The paper introduced a method to segment the area of longissimus dorsi and extract marbling from ribeye imaging based on mathematic morphology. Pretreatment of ribeye imaging was obtained after removing the background color and separating fat pixel from muscle pixel, and the morphology filter was used to separate all objects clearly. After then the area of longissimus dorsi was obtained by repeated using the operation of dilation and erosion. The marbling was extracted by using logic operation.