The concept of relative decision is put forward and the method for finding absolute reduction decision rule through combining rules of relative decisional condition sets is given in this paper. In the process of decision table reduction, the relative Discernibility Matrix between decision categories should be created in order to decide the relative decision conditions which are conjoined to find the decision conditions of all the decision classes. The relative decision conditions are extracted by creating relative Discernibility Matrix, so that scales of computing can be cut down substantially and saving storage space. The availability of this method is proved by a fault diagnosis example of reciprocating machines.