Calcium carbonate was surface-modified by plasma-polymerized acetylene and the effect of surface modification on the mechanical properties of calcium carbonate-polypropylene composites was investigated. Two different plasma polymerization conditions were selected and applied. Chemical structures of plasma-polymerized acetylene products were identified. Mechanical and thermal properties of the composites prepared were evaluated and the effects of surface modification on the extent of adhesion of filler to the matrix, as well as on polymer phase, were investigated with the help of scanning electron microscopy. Some of the composite samples prepared with surface-modified calcium carbonate are found to yield higher percentage elongations and are mechanically superior compared to those prepared with unmodified filler.