In this paper, the concept of a maximal consistent block is applied to formulate a new approximation to an object set in incomplete information systems with higher level of accuracy. Similar to the method in [Inform. Sci. 112 (1998) 39, 113 (1999) 271], the proposed rough-set-based rule acquisition method does not require change in the size of the original incomplete system. It, however, has the additional advantage of using a set of simpler discernibility functions of an incomplete system. This means that it can provide a more efficient computation for knowledge acquisition, especially in large incomplete systems.