The resultant thermoelectric power factor P and figure of merit Z of two types of composite device composed of a sandwich structure (A/B/A) were calculated by treating these devices electrical and thermal circuits. When the direction of the temperature gradient is perpendicular to a sandwiched slab with a lower rho and a higher kappa than those of the dominant material (a) and is parallel to a slab with a higher rho and a lower kappa than those of the dominant material (b), P increased significantly at an optimum slab thickness for device (a), in accordance with the result obtained by Bergman and Fel for a similar composite device, but decreased abruptly with increasing slab thickness for device (b), while Z remained almost unchanged with slab thickness for both devices as long as a thin slab is used. It was clarified that well-known high-performance thermoelectrics have crystal structures or microstructures corresponding to either device (a) or (b) fitted to enhance the boundary effect at the interface. Therefore, it is expected that when a number of thinly layered phases aligned in one direction are introduced into the microstructures of high-performance bulk materials; they enable the significant enhancement of boundary effect alone, resulting in a significant increase in Z of such bulk materials.