Abstract: The result of Gibson's formula that describes equivalent elastic parameters of cellular material is simple and applied easily. However because the stiffness corresponding with extension or contraction deformation of honeycomb wallboard is neglected, it causes indeterminedness shown by the elastic matrix of material of core in numeral analyzing honeycomb sandwich panels. In this paper it is again discussed that the influence of extension or contraction deformation of honeycomb wallboard on in-plane stiffness...