Abstract: The problem occurred frequently in the course of process reasoning is how to select the most appropriate rule from multiple optional rules. With the help of the trait that Rough set theory can effectively resolve uncertain problems, several granularity formulas used to implement measurement of process rules are introduced, and a feasible formulation on the distance between the varieties of rules is put forward. Based on analyzing traditional conflict resolution in process decision, an approach to rule set reduction in the course of process decision reasoning is presented. The practicality and efficiency of this method are demonstrated by the analytic results of the example.