The mechanical properties of multifilament yarns, spun from the blends of a plastic-grade polymer with a fiber-grade CR-polymer in the composition range of 10-50 wt % added, were investigated. The predicted modulus of a two-phase blend, calculated from several representative equations, was compared with the elastic modulus of drawn yarns, determined from the stress vs. strain curve and dynamic modulus obtained from the sound velocity measurements. The best tit was achived with the Kleiner＇s simplex equation. For both the static and dynamic elastic modulus, the largest negative deviation is seen at the 80/20 and 60/40 plastic/fiber-grade polymer blend composition, while the largest positive deviation is seen at the 90/10 plastic/fiber-grade polymer blend composition, suggesting good compatibility of both polymers, when only a small percent of the fiber-grade CR-polymer is added. Improved spinnability and drawability of blended samples led to the yarns with the tensile strength over 8 cN/dtex, elastic modulus over II GPa and dynamic modulus over 15.5 GPa. Structural investigations have shown that the improved mechanical behavior of blended samples, compared to the yarn spun from the pure plasic-grade polymer, is the consequence of a higher degree of crystallinity, and above all, of a much higher orientation of macromolecules. (C) 2000 John Wiley & Sons, Inc.