Abstract: Inference leakage problem in high security database is an important subject to research. Analysis of inference channel is the basis of solving inference leakage problem. In this paper, a more sound inference rule about inference channels, namely weak functional dependency of equivalence (referred to as WFDoE), is proposed and its formal definition is also presented. Rough set theory is used to define WFDoE's the degree of credibility and support, a mining arithmetic of WFDoE inference rules from database is provided. Experimental result shows that the proposed arithmetic is able to enhance ability to prevent inference leakage.