Abstract: The vertical density profile, density gradient and interface of three types of plywood, namely homogeneous, thin shell and inner strengthened structures, were investigated in this study. Strain distributions of different gradient density interfaces before and after the aging process were measured by the digital speckle correlation(DIC)method. Experiment results showed that the density of the homogenous structure plywood was relatively uniform, measuring about 0.83 g/cm³. Large density gradient and weak interface existed on the cross sections of both shell and inner strengthened structure plywood. Under a load of about 450 N, the maximum shear strain occurred near the notches at both ends of the bonding interface and shear strain decreased along the bond line to the middle zone. Generally, the shear strain of the higher density layer of gradient interface was greater than that on the lower density layer of the specimens. After aging, the shear strain on the bonding interface decreased compared with the control group. The higher density layer of the gradient interface of specimens had greater resistance to aging process, which was resulted from its slower strain decrease than that on the lower density layer on the gradient interfaces.