The resultant thermoelectric properties of the p- and n-type Ag/Bi-Te/Ag composite devices welded with pure Bi were measured at 298 K as a function of relative thickness of x, where x is the ratio of thickness of Bi-Te compound to the interval between two thermocouples and the chemical compositions of the p- and n-type Bi-Te compounds used here are (Bi0.25Sb0.75)(2)Te-3 and Bi-2(Te0.94Se0.06)(3), respectively. Consequently, the electrical resistivities rho of the p- and n-type Ag/Bi-Te/Ag devices increased linearly with an increase of x, while the Seebeck coefficients alpha were enhanced significantly in the range from x = 0.03 to 0.10, so that their observed P values have a large local maximum at x = 0.06. The x-dependence of P values was found to be explained roughly with the simple model proposed here when some reduction in the thermal conductivity kappa of Ag and Bi was taken into the calculation. The maximum P of the p- and n-type Ag/Bi-Te/Ag devices reached extremely large values of 27.8 and 88.3 mW/K(2)m, which are higher than 25.7 mW/K(2)m obtained for the previous n-type Ni/Bi/Cu device. (c) 2006 Springer Science + Business Media, Inc.