Ziegler-Natta isotactic polypropylene (ZN-iPP) and metallocene isotactic polypropylene (m-iPP) were extruded (in ratios of 75/25, 50/50, and 25/75) from one melt twin-screw extruder to produce three ZN-iPP/m-iPP polyblended polymers and, subsequently, spin fibers. In this study, we examined the rheology of the ZN-iPP/m-iPP polyblended polymers and the thermal characteristics and crystallinity of the ZN-iPP/m-iPP polyblended fibers using gel permeation chromatography, rheometry, differential scanning calorimetry (DSC), wide-angle X-ray diffraction, density gradient analysis, and extension stress-strain measurement. The apparent melt viscosity of the ZN-iPP/m-iPP polyblended polymers revealed positive-deviation blends. The 50/50 blend of ZN-iPP/m-iPP had the highest apparent melt viscosity. For five samples, the complex melt viscosity decreased with the angular frequency, which represented typical non-Newtonian behavior. The Cole-Cole plot, which consisted of the imaginary part of the complex melt viscosity versus the real part of the complex melt viscosity plot, of the ZN-iPP/m-iPP polyblended polymers showed a semicircular relationship with the blend ratios. It indicated that the ZN-iPP/m-iPP polyblended polymers were miscible. We analyzed the shear modulus data (G' vs G '') by plotting them on a log-log scale. The plot revealed almost the same slopes for the ZN-iPP/m-iPP polyblended polymers, which indicated a good miscibility between the ZN-iPP and m-iPP polymers consisted a miscible system. The crystallinity and tenacity of the ZN-iPP/m-iPP polyblended fibers initially increased and then fell as the m-iPP content increased. Meanwhile, the 50/50 blend of ZN-iPP/m-iPP had the highest crystallinity and tenacity (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 265-173, 2009).