The purpose of this note is to give a version of Clifford Theory for Schur subrings of Schur Rings denned over a field and to examine the rings of invariants for group actions on the group rings. The expression of the central primitive idempotent, relating to an irreducible character of a Schur ring as well as a characterization of Schur rings corresponding to a partition of sets with maximal length two, is given.