Abstract Temperature and performance modeling of thermoelectric generators (TEGs) have long been discussed. However, due to the high nonlinearity of heat conduction in thermoelectric (TE) materials, the analytical model of TEGs is difficult to develop and there is no such model exists at this time. In this paper, we revisit the problem and develop an analytical model to evaluate the performances of a TEG. The model considers contact resistance between TE leg and electrode, and temperature dependent TE material properties. Through the analytical model, we give simplified expressions to calculate the leg temperature profile, energy conversion efficiency, and output power for a single TE leg pair. These expressions are validated by either experiment or finite element (FE) simulation. The results show that an optimal cross-section area exists for maximum efficiency. The optimal leg length decreases as the thermal contact conductance increases while the optimal leg length has no relation with electrical contact resistance. Based on the results of this paper, we provide some useful suggestions for the design of high-performance thermoelectric generators.