Abstract: Attributes value reduction is one of the important parts researched in rough set theory.In order to correct the errors of computing reduction of the values in a decision table based on discernibility matrix,this paper puts forward an improved discernibility matrix definition together with a method for values reduction.It takes the new inconsistency into consideration caused in the values reduction.Therefore,the matrix discerns the rules that have the same values of decisive attributes with the inconsistent rules in order to avoid the possibility of including wrong rules after values reduction.