Abstract: In order to compute the minimum set of rules of decision table when a new instance is given into a Knowledge Representation System for incremental learning, all the data in the decision table will be recalculated in the classical method. Clearly, this method is not effective. In this paper, a criteria for the minimal recalculation based on the rough sets theory is given, and an improved algorithm for incremental learning is present. The improved algorithm in this paper is better than the classical method in some sense.