Recently, a bentonite sheet is often employed in deteriorated earth-fill dams for preventing water leakage. However, it is concerned that a slipping type of failure may occur at the boundary between the bentonite sheet and the fill material in the event of seismic attack. In this paper, seismic behavior of a small-sized earth fill model inside which a bentonite sheet was mantled was examined in shaking table test, and it was simulated by numerical analysis. It is manifested in the shaking table test that a shallow slip surface developed in the fill above the bentonite sheet, and no harmful deformation was observed along the bentonite sheet when subjected to seismic motion, noting also that the shallow failure was successfully simulated by pseudo static limit equilibrium analysis. In addition, it was confirmed that the seismic response of the model earth-fill was unaffected by the existence of bentonite sheet as examined by FEM seismic numerical simulations.