In order to improve the performance of semiconductor thermoelectric power generators, a high figure of merit Z of both material and generator system has been a target for a long time. In this technical brief, it is demonstrated that a higher value of Z does not always guarantee better generating performances because of the thermoelectric-coupled effect in the generators. Based on a coupled analytical thermal model and a set of performance formulas with considerations of heat transfer irreversibility, numerical simulation examples are provided to support the analysis, by which some important relations between design parameters and performance parameters are described.