Calcium carbonate (CaCO3) was crystallized in the presence of O-carboxymethylchitosan (CMCS) by a vapor diffusion method. The CaCO3 particles were characterized by scanning electron microscopy (SEM), the energy dispersion spectroscopies (EDS), powder X-ray diffraction (XRD). Fourier transform infrared (FTIR) spectroscopy, thermogravimetry analysis (TGA) and N-2 adsorption-desorption methods. By varying the concentrations of CMCS. Ca2+ and Mg2+ ions, and temperature, the CaCO3 nanocrystals with different morphologies were produced. Based on the analysis of CaCO3 particles obtained under different conditions, the mechanism of the CaCO3 crystallization in the presence of CMCS was proposed. Crown Copyright (c) 2010 Published by Elsevier B.V. All rights reserved.