Abstract: In the classical rough set theory, d R（F） and r R（F） are used to describe the relative knowledge capacity of two knowledge base This description is relative and inexact In this paper,according to the difference degree of knowledge in structure, the similarity relation of knowledge base is defined as the equivalence, equal structure and equal potential relation Then, on the basis of the above three relations, the four criterions which define the measure of the knowledge capacity of knowledge base are proposed The meantime, two methods which measure the knowledge capacity of knowledge base are given and validated