Gelatin was modified by 2,3-epoxypropyltrimethyl ammonium chloride (EPTAC), and the EPTAC modified gelatin (EPTAC-m-gelatin) was used to modulate CaCO3 crystallization by a slow gas diffusion method. The influences of concentrations of EPTAC-m-gelatin and Ca2+, reaction temperature and aging time on the crystallization of CaCO3 were investigated. The CaCO3 particles were characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), powder X-ray diffraction (XRD), and thermo-gravimetric analysis (TG) methods. We found that the electrostatic interaction between EPTAC-m-gelatin and Ca2+, the intra- and inter-molecular interactions between EPTAC-m-gelatin molecules, and the adsorption of EPTAC-m-gelatin molecules on CaCO3 particles played a key role in the crystallization of CaCO3 with different morphologies and polymorphs. Concentrations of EPTAC-m-gelatin and Ca2+, temperature and aging time showed different influences on CaCO3 crystallization and growth. Based on the analysis of the CaCO3 particles, a mechanism of CaCO3 crystallization in EPTAC-m-gelatin solution was proposed. The work would provide new insight into the CaCO3 crystallization process in the presence of polypeptide solutions. (C) 2017 Elsevier B.V. All rights reserved.