A rheological technique of determining molecular weight distribution (MWD) of ultra high MFR polypropylene (PP) is presented. The development of this new technique was needed for two reasons. First, the advent of high activity catalyst made possible the direct polymerization and thus the commercialization of very high MFR PP. Secondly, the polydispersity index (PI) measurement based on crossover modulus from G＇ and G＇＇ curves, which was developed by Zeichner and Patel in 1981, was proven to be inapplicable to resins with MFR higher than about 40, due to instrument limitations. This new technique utilizes G＇ and G＇＇ data at low frequencies which are obtained from a dynamic frequency sweep test at 200-degrees-C. PI values, a measure of MWD, determined by this method are proven to have an excellent correlation with those obtained by gel permeation chromatography (GPC). This method has been very useful in determining MWD of melt brown fiber resins whose MFR is normally between 30 and 1000. It has also been used as a quality control tool during the resin production in a commercial plant.