Two identity atoms initially in a maximally entangled state, the fields in the singlemodel thermal fields, and the TavisCummings (TC) model are studied. Effects of Stark shift added in the Hamilton of the two identity atoms TC system on the decoherence time of atomic entanglement are analyzed. The results show that the two atoms evolve periodically into a disentangled separable state when Stark shift is not considered; the two atoms keep longlived entanglement when Stark shift is considered.