Empirical formulas, which describe the air velocity and air temperature decays along the spinline of a dual slot die, are improved by introducing the influence of the ratio of die end width to air slot width. The thus-improved empirical formulas include more die geometry parameters and are more accurate than the existing ones. The fiber diameters of nonwoven web are predicted by incorporating these new empirical formulas into a one-dimensional theoretical model for melt-blowing process. The agreement between the predicted results and the experimental data cited from literature is very good, which confirms the reliability and the accuracy of these new formulas.