This paper presents a detailed empirical formula on the band gap narrowing of strained Si1-xGex base as a function of impurity concentration, temperature and Ge content x. This method has been shown to be very accurate, and has been applied to strained Si1-xGex base on (0 0 1) Si substrates. We have calculated the band gap narrowing contribution of the germanium, high doping effects and temperature based on the empirical relation. The values obtained are very comparable with the theoretical and experimental results given by the literature.