The development of thermal stress during the diffusion bonding of Al2O3 ceramic to Al is analyzed by the finite-element analysis using MARC/MENTAL code. Prior to diffusion bonding, a Cu layer of 0.2mm in thickness is sintered on the ceramic. The results show that shear stress concentration occurs in the ceramic near the corner of Al2O3/Cu interface, and tensile stress concentration occurs near the corners of Al2O3 outer surface during cooling from bonding temperature to room temperature. After tensile stress concentration on the Al2O3 undersurface rises to a certain value, ceramic deforms gradually, resulting in stress beginning to relax. The stress relaxation originates in the stress concentration region of Al2O3 outer surface.