Abstract: Spherical vaterite CaCO3 nanoparticles with hydrophobic properties were synthesized in situ in the presence of oleic acid as surface modifier via double decomposition method. The products were characterized by scanning electron microscopy（SEM），X-ray diffraction（XRD），Fourier transform infrared spectroscopy（FT-IR），and contact angle analysis.The effects of calcium chloride concentration and ethanol content on calcium carbonate nanoparticles were analyzed.Results demonstrated that the raising of the calcium chloride concentration and ethanol content was advantaged to prepare CaCO3 powder with smaller size and better disperse performance，but had little effect on the crystal form and morphology.Furthermore，it was analyzed that the surface properties of the vaterite CaCO3 nanoparticles changed from hydrophilic to hydrophobic.The reason of surface modification was also discussed.

Key words: double decomposition method, synthesis in situ, vaterite, calcium carbonate nanoparticle