In the case of bipartite two-qubit systems,we derive an analytical expression of bound Bell operator for any given pure state.Our result not only manifests some properties of Bell inequality,for example,which may be violated by any pure entangled state and only be maximally violated for a maximally entangled state,but also gives the explicit values of maximal violation for any pure state.Finally we point out that any mixed states which can produce maximal violation of Bell inequality must have a maximal concurrence value.