A method for differentiating dynamic range of image is disclosed. The method comprises the following steps. First of all, pixel numbers with the same level value of Density units D.sub.n-1 and D.sub.n are compared. Next the minimum pixel numbers of each level value of Density units D.sub.n-1 and D.sub.n are counted. Then the minimum pixel numbers of each level value of Density units D.sub.n-1 and D.sub.n are summed. Next the ratio of total minimum pixel number and total pixel number R and a specification value R.sub.s are compared. Then whether R is smaller than R.sub.s or not is determined so as to recognize whether Density units D.sub.n-1 and D.sub.n could be differentiated or not.
