Abstract: In this paper, in-plane equivalent elastic modulus of ahoneycomb sandwich, consisting of hexagonal cell core and face sheet, isstudied by using a theoretical analysis. It is shown that the equivalentelastic modulus can not been analyzed with high precision by treating thehoneycomb as a composite of two materials: the core and the face sheet,because the core consists of portions of the slanting cell walls and thevertical cell walls, whose rigidities differ greatly. Dividing the core intothe slanting cell wall portion and the vertical cell wall portion, andregarding the honeycomb as a composite of three materials, a method tocalculate elastic modulus is proposed based on the principle of the minimumpotential energy, and its validity is verified by numerical results of FEM.