A possible wave effect in the gravitational lensing phenomenon is discussed. We consider the interference of two coherent gravitational waves of slightly different frequencies from a compact binary, due to the gravitational lensing by a galaxy halo. This system shows the modulation of the wave amplitude. The lensing probability of such the phenomenon is of order 10 −5 for a high-z source, but it may be advantageous to the observation due to the magnification of the amplitude.The wave effect in gravitational lensing phenomenon has been investigated by many authors (see e.g., [1,2] and references therein). This subject is recently revisited by several authors, motivated by a possible phenomenon which might be observed in the future gravitational wave experiments [3][4][5][6][7][8][9][10][11][12]. In these works, the authors focus on the diffraction in the wave effect, which is substantial for λ ∼ R E , where λ is the wave length and R E is the Schwarzschild radius of the lens mass. However, in the present work, we focus on another different aspect of the wave effect, the modulation of superposed two waves, in the limit that the geometrical optics is valid λ ≪ R E . Some aspect of this effect has been investigated by the author and Tsunoda [13] as a lensing phenomenon by a cosmic string. Here we perform the similar analysis for the lensing by a galaxy halo. Throughout this paper, we use the convention G = c = 1.We start by considering the superposition of two lensed waves with the amplitude A 1 and A 2 and the angular frequencies ω 1 and ω 2 . Namely, we consider the wave expressed bywhere we assume that the phase δ involves the information of the path difference, and the difference of the frequencies ω 1 and ω 2 are due to the time delay effect. Assuming A 2 = A 1 + ∆A and ∆A << A 1 , E reduces toThis means that, if ∆A/A 1 ≪ 1, the wave amplitude modulates with the periodwhere ∆T is the time delay and dω/dt is the change rate of ω for an observer. We consider a gravitational wave from a binary of compact objects with equal mass M . By decreasing energy due to the gravitational wave radiation, the orbit of the binary changes. Thus the angular frequency ω changes. The rate of the change is estimated as [14],where z s is the redshift of the source. Here note that ω and t are the angular frequency and the time of the observer. Now we consider a lens halo modeled by the singular isothermal sphere with the velocity dispersion σ. Then the time delay is ∆T = 2.7 × 10where D OL , D OS and D LS are the angular diameter distances following the usual convention [2], f r is the flux ratio of the two waves, z l is the redshift of the lens, and we have adopted the Hubble parameter H 0 = 70km/s/Mpc. Then the period of the modulation is estimated as 1