A new hyperdispersant agent with Si-OH as an anchoring group and poly(butyl acrylate) as a solvatable chain was synthesized, and its effect on the properties of polypropylene (PP)/CaSO4 composites was investigated. Fourier transform infrared spectroscopy results showed that the hyperdispersant agent reacted on the CaSO4 surface and the modified CaSO4 particles. The tensile strength and impact strength of the PP/CaSO4 composites increased about 14 and 34%, respectively, versus that of PP/CaSO4 (filled with the same unmodified fraction). According to surface analysis by scanning electron microscopy, the CaSO4 particles were buried well in the PP matrix when CaSO4 was coated with the hyperdispersant agent. CaSO4 significantly increased the crystallization temperature and crystallization rate of PP by differential scanning calorimetry, but the addition of hyperdispersant-agent-modified CaSO4 did not lead to the formation of crystalline PP through X-ray diffraction. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 532-538, 2009