In this article, blends of polypropylene random copolymer (PP-R) with a novel impact modifier, namely ethylene/styrene interpolymer (ESI), were prepared to evaluate the effectiveness of ESI in toughening PP-R and the influence of ESI content on the mechanical, thermal, and rheological properties of polymer blends. Results showed that super-toughened PP-R/ESI blends (ca. Izod impact strength >= 500 J/m) were readily achieved with only 5 wt % ESI. The blends exhibited significant improvement in both impact strength and elongation, while small loss in tensile strength and elastic modulus when increasing ESI content. ESI had a nucleating effect that caused PP matrix to crystallize at higher temperatures, whereas PP-R/ESI blends presented lower melting temperatures (T-m) than PP-R matrix and T-m decreased with the increment of ESI content. Rheology study indicated that both PP-R matrix and PP-R/ESI blends presented shear thinning behaviors during melt processing. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 115: 190-197, 2010