This paper describes a new way for measuring the apparent band gap narrowing in bipolar transistors with Si base or epitaxial Si0.80Ge0.20 base from the temperature dependence of the collector current Jc(T). A graph of ln J(C) as a function of V-BE is plotted based on the data extracted from the linear region of the Gummel plot as the temperature is kept as a constant. We have obtained two lines of InJ(c)-V-BE at temperature, 300 and 77 K, respectively. From the intersection point of the lines we can calculate the values of band gap narrowing in Si and Si0.80Ge0.20 bases, which are 41 and 151 meV, respectively. The results are in good agreement with the measured values in reference.