In this paper, in-plane equivalent elastic modulus E _y of a hexagonal honeycomb sandwich, which includes the effect of face sheet interference, is studied by using numerical results of FEM. It is shown by comparing with deformation of practical honeycomb sandwich that there are two error factors to apply the rule of mixture to honeycomb sandwich. One of error factors is that the deformation of honeycomb core does not coincide with the face sheet since an inclined cell wall deforms much larger than a vertical cell wall. Another one is that the non-uniformity deformation of core along the height direction is induced by the interference effect with face sheet. Then, a method to calculate elastic modulus based on the compatibility condition of core and face sheet is proposed, and its validity is verified by using numerical results of FEM.