We developed a structure of a mechanical-strength-enhanced low-temperature co-fired ceramic (LTCC) substrate using two LTCC materials with different thermal expansion coefficients (TECs). In this structure, the top and bottom surface layers are made of a material with a lower TEC and the internal layers are made of another material with a higher TEC. Because compressive stress was thought to be exerted in the surface layer due to the differences in the TECs between two materials, it was expected that the compressive stress would reinforce the surface. We found that the bending strength of the substrate was more than 400 MPa and considerably higher than that of each material. The results of stress analysis using the finite element method indicate that the fracture stress of the internal layer considerably exceeded the mechanical strength of the material comprising the internal layer. This is thought to be the cause of mechanical strength enhancement. Therefore, this proposed structure is promising for thinner multilayer substrates.