Abstract: In this paper, dynamic behavior of orthotropic honeycomb sandwich plates is investigated layer by layer with high order theory of Reddy. Dynamic equations of honeycomb sandwich plates are derived. Free vibration of one dimensional orthotropic honeycomb sandwich plates is investigated deeply. Frequency equations and functions of vibration mode are given when two sides, opposite to each other, are simply supported. How frequency is influenced by thickness of core and ratios of thickness to span is analyzed simultaneously. The results are compared with those of low order theory. The result of comparison proves that high order theory is better than low order theory.