In this paper we study the properties of idempotent uninorms on the lattice, that are one of the binary operations. It is shown that in any lattice idempotent uninorms need not be internal (with the extended definition of the term "internal"). But with additional assumptions, we get that the uninorm is locally internal. With this assumption, we present the theorem of Czogala and Drewniak for a complete lattice. Moreover, many properties of idempotent uninorm in this case is shown.