O-(hydroxy isopropyl) chitosan (HPCHS) and cetyltrimethylammonium bromide (CTAB) were employed as the modulation agents for CaCO3 crystallization. The effects of HPCHS and CTAB concentrations, as well as aging time on CaCO3 crystallization were investigated. The CaCO3 particles were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), powder X-ray diffraction (XRD), thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FTIR). All the crystallized particles are aggregated rhombohedral crystals with rough surfaces. HPCHS and CTAB show different effects on the crystallization and growth of CaCO3 particles. Based on an analysis of the CaCO3 particles and solutions in the HPCHS/CTAB system, a CaCO3 crystallization mechanism in HPCHS/CTAB mixed solutions is proposed. The work provides good insight into the CaCO3 crystallization process in the presence of biomacromolecule/surfactant mixed solutions. (C) 2014 Elsevier B.V. All rights reserved.