Abstract: A new kind of fiber acousto-electro-optic (AEO) modulator by using lithium niobate (LN) crystal is made. This device has an additional direct-current electrode compared with traditional fiber acousto-optic (AO) modulator. Its center frequency can be changed by electro-optic (EO) effect so that the use of the device be more flexible. The AEO effect includes AO effect and EO effect at the same time. The AO effect is used for light modulation and light deflection. The formula of diffraction efficiency of the AEO effect is given from a couple-wave equation of the AO effect. The optimum modes of the AEO effect are discussed. The geometric relations of anisotropic AO interaction in LN are calculated and a LN fiber AEO modulator is designed and made. Some curves of relative diffraction efficiency vs power of electric signals driving transducer and direct-current voltages driving electrode are given respectively. The experiment results are agreed with the theory.