The tensile properties of poly(vinyl chloride) filled with precipitated calcium carbonate particles having two different mean sizes were studied. The particles were porous agglomerates of spherical primary particles. For this purpose, 10-50 parts of the particles were mixed with 100 parts of poly(vinyl chloride) and 3 parts of lead stearate using a mixing roll. The particle size decreased during the mixing process. The measured mean sizes in the resin were about 0.6 and 1.2 mu m at filler contents of 10 phr. The tensile tests were carried out using dumbbell specimens. The yield stress decreased only slightly with filler content in the small particle-filled system, whereas it decreased considerably with an increase in filler content in the large particle-filled system. From a scanning electron microscopic observation of the specimen surfaces after test, the larger particles used in this study seemed to be more brittle, therefore, the particles tended to fracture when a stress was applied, and voids formed around them. These observations appear to explain the decrease in yield stress in the large particle-filled system, because the voids made the ductile polymer matrix yield around the particles.