The output power and conversion efficiency of the thermoelectric generator (TEG) are closely related to not only the materials properties but also the geometric structure. This paper developed a multi-physics, steady-state, and three-dimensional numerical TEG model to investigate the TEG performance, and then the model is compared with the classical thermal resistance model. Bismuth-telluride are used as p- and n-type materials. The comparison reveals that the assumption of constant material properties leads to underestimated inner electrical resistance, and overestimated thermal conductance and Seebeck coefficient, so that the thermal resistance model predicts unrealistically high performance than the present model. The results also indicate that when heat losses exist between the TEG and the ambient, although the output power is slightly elevated, the conversion efficiency is significantly reduced, hence, improvement of the heat insulation effect is critically important for high-temperature TEGs. Furthermore, the TEG geometry also affects its performance significantly: usage of thin ceramic plates increases the junction temperature difference, and hence enhances the TEG performance: there are two optimal leg lengths which correspond to the maximum output power and the maximum conversion efficiency, respectively; when heat losses are not ignorable,,a large semiconductor cross-sectional area remarkably reduces the ratio of the heat liberated to the ambient to the heat absorbed from the high-temperature heat source, and hence improves the conversion efficiency. (C) 2014 Elsevier Ltd. All rights reserved.