AbstractThe Rheometrics mechanical spectrometer has been employed in the parallel plate mode to study the rheological behavior of pure polypropylene resin and melts filled with 10, 20, 30, and 40 percent by weight glass fibers. Steady‐state shear data show that, for loadings above 20 percent fiber, increasing fiber loading leads to an increase in viscosity of the melt. The effect of increasing fiber content becomes smaller at higher shear rates. For the same loading range, fiber addition increases the first normal stress difference, and the effect of fiber loading becomes smaller with increasing shear rate. At 10 percent loading, the viscosity of the filled polymer melt is lower than that of the pure resin melt. These results require further confirmation.Strain sweeps employing dynamic measurements in conjunction with the parallel plate mode of the Mechanical Spectrometer were carried out. In general, it was found the dynamic properties are strain dependent. Steady shear data employing the cone‐and‐plate mode were not repea