Abstract: The definitions of effectively equivalent class subdivision and effective set subdivision are given to the decision system by rough set theory. The influence of the necessary attributes after class subdivision on the upper and lower approximate values, quality and accuracy of approximate classification, number of decision-making rules and relative reduction are analyzed theoretically. Three cases concerning different subdivision regions are discussed with relevant results obtained. It is theoretically revealed that the finer the effective set subdivided, the closer the lower and upper approximate values to the original set and the higher the quality and accuracy of approximate classification and, at the same time, the more the number of the decision-making rules increased. In addition, the relative reduction and the necessity and unnecessity of attributes all remain unchanged. The results revealed provide an actual significance to the attribute reduction, formation of decision-making rules and their effectiveness.