An externally and internally irreversible heat engine model of thermoelectric generators is used to analyze the so-called device-design parameter introduced by Ozkavnak et nl. The simple expression of the parameter is given and its physical meaning is expounded. Moreover, the optimal range of the parameter is determined and the problems relative to the maximum power output and maximum efficiency are discussed. Some meaningful results are obtained. [S0195-0738(00)00401-5].