It is well-known to us that the Pawlak’s rough set theory, an effective tool to deal with uncertainty and granularity in information systems, is based on equivalence relation. However, in some situations, those conditions of equivalence relation are hardly met. In this paper, we define a new type of upper and lower approximations of rough set in partially ordered sets and propose the reducible element in this situation. In the end, we prove some properties of rough set in the partially ordered sets which can form a covering.