Polypropylene (PP) was modified with elastomer or CaCO3 particles of two different sizes (1 mum and 50 nm) in various volume fractions. The dispersion morphology and mechanical properties of the two systems were investigated as functions of the particle size and volume fraction of the modifier. The brittle-to-tough transition occurred when the matrix ligament thickness was less than the critical ligament thickness, which was about 0.1 mum for the PP used here, being independent of the type of modifier. At the same matrix ligament thickness, the improvement of the toughness was obviously higher with the elastomer rather than with CaCO3, but adding CaCO3 increased the modulus of PP. (C) 2004 Wiley Periodicals, Inc.