Given a fuzzy variable,what is the degree of difficulty of predicting the specified value that fuzzy variable will take? Entropy provides a measure to characterize the fuzziness.As we know,image segmentation using fuzzy entropy is an important and common segmentation method.The threshold of fuzzy entropy is mostly selected at the gray value with fuzzy membership degree 0.5.Compared with the tradition fuzzy entropy-based image segmentation,this paper provides a personal overview of entropy of S-function,and get a correspondingly symmetrical result.Our result will be a good supplement to the fuzzy entropy-based image segmentation method.