By combining the modified HR variational principle for elastic material with the quadratic interpolation functions, the formulation of isoparametric element with 8node for Hamilton canonical equation in the planecoordinate was derived. The continuity of stresses and displacements on the interface between layers was applied to separate into top/bottom layers.By using the separatingcombiaing method, the state equation with delamination situation was established. The natural frequencies of delaminated plate were studied and the numerical examples show that the method presented is correct.The method of using isoparametric element with 8node for Hamilton canonical equation reduces the number of nodes, and improves the efficiency.