Abstract: Assessment of image quality is an important problem in image processing，but it is very difficult to give an objective evaluation．A definition of the fuzzy degree of digital image is presented by introducing some concepts in fuzzy mathematics，such as triangular norm operator and non-fuzziness cardinality．Being proved both in theory and in experiments，this definition is in better accordance with human subjective vision than that of fuzziness index and can be used to assess the image quality more reasonably．The fuzzy degree of image can distinctly raise the efficiency of image processing systems．