The maximum output power of a thermoelectric module ( TEM) can be estimated according to the opencircuit voltage and the short circuit current under certain temperature difference. As there are two switch modes,namely the mode from open to short circuit and the mode from short to open circuit,there may exist two groups ofopen circuit voltage and short circuit current.Consequently,under identical temperature difference,the maximumoutput powers of TEM in the two modes are different from each other,and the difference varies with the temperaturegap between the hot and the cold sides.This paper deals with such power difference based on theoretical analysisand points out that the power difference is due to the variable electromotive potential caused by the current varyingwith the switch mode.Moreover,a computation method to reduce the power difference is put forward.Experimentalresults show that the proposed method greatly improves the estimation accuracy of maximum output power of TEM.