Abstract: A new dominance relation is defined in consistent set-valued information system based on similarity grade of attribute value sets. The problem of attribute reduction and the judgment in consistent set-valued decision information system are studied, from which a new approach to attribute reductions is provided in consistent set-valued decision information system. The assignment reduction is introduced in inconsistent set-valued decision information system, and it is shown that the inconsistent system can be turned into a generalized consistent approximation space. Furthermore, the influence of similarity level on attribute reduction is discussed