Knowledge and classifications are related together by the theory of rough sets which claim that knowledge is deep-seated in the classificatory abilities of human beings.In this paper,quantitatively represent the ability of knowledge’s classification by partition granularity.Firstly,the relationship between knowledge and its partition granularity is set up.Secondly,the significance of attributes is defined from the view of partition granularity,and a heuristic algorithm based on partition granularity for reduction of an information system is proposed.Finally,shows that this algorithm is effective for dealing with relatively large-scale information system through an example.