The relationship between journal impact factor and paper quality，and the imbalance distribution of impact factors among different subjects are discussed．The highly skewed distribution of impact factors among journals，and among disciplines is studied．Ranking journals hy impact factor，we divide the source journals of the SCI and the CSCD into different Bradford zones．This leads to an unified weighting scheme for scientific jouraals．The use of this new unified scoring method is illustrated on the top 10 universities and research institutes that have the highest score in China．