A method of automatically grading beef quality by analyzing a digital image is provided. The method includes: an image acquiring step of acquiring a color image of beef using a CCD camera; a region separating step of separating a lean region from the acquired image; a boundary extracting step of extracting a boundary line of the lean region; a boundary smoothing step of smoothing the boundary line extracted in the boundary extracting step; a boundary correcting step of correcting an indented portion and a protruded portion included in the boundary line having been subjected to the boundary smoothing step; a grading region determining step of determining a grading region on the basis of the boundary line corrected in the boundary correcting step; and a grading step of grading the beef quality on the basis of the image of the grading region.