The mechanisms of deformation and fracture of isotactic polypropylene filled with CaCO3 particles were studied. Three types of particles with average diameters of 0.07, 0.7, and 3.5 mum were used at filler volume fraction from 0.05 to 0.30. The experiments included slow tensile tests, notched Izod impact tests with varying notch depths, and fracture resistance tests using double-cantilever-beam sample configurations. In slow tension, addition of fillers increased the modulus and decreased the yield stress independently of filler type. The strain at break increased with initial incorporation of fillers but decreased at higher loadings. The 0.7 mum diameter particles improved Izod impact energy up to four times that of the unfilled matrix. The other particles had either adverse or no effect on the impact toughness. The toughening mechanisms at work were plastic deformation of interparticle ligaments following particle-matrix debonding with additional contribution coming from crack deflection toughening. The failure of the 0.07 and 3.5 mum diameter particles to toughen the matrix was attributed to poor dispersion.