Abstract: A computational model,Tri-layer Unit Cell(TUC)model,which can reveal the out-plane size variation of sandwich panel unit cell,is proposed to study the out-plane size effects of honeycomb sandwich panels.In this model,based on homogenization method,the three dimensional unit cell of sandwich panel consisting of the upper and the lower skins and the homogenized core is constructed.It is compared with three methods,i.e.,homogenization method,the finite element method and the classical laminated plate theory,to illustrate influences of the out-plane size variation on bending and vibration response of sandwich panel.Numerical results show that the proposed TUC model can characterize the out-plane size effects of sandwich panel with different configuration cores.Significant differences between the homogenization results and the solutions by the other two methods demonstrate the limitations of the homogenization method when the sandwich panel contains no more than three cell layers in out-plane direction.