The microstructure of CaCO₃ nanoparticles with different particle sizes was investigated by SEM, X-ray diffraction and FTIR spectra. Results showed that CaCO₃ nanocrystal possesses the scale effect and the distortion stress exists in the crystal structure. The abnormal infra-red behaviors of nanoparticle calcite, including the blue shift and obvious narrowness of the ν₃ assimilation band, were due to the defect and strong distortion stress of the crystal lattice in CaCO₃ nanoparticles. The activation energy of the thermal decomposition of CaCO₃ decreases with decreasing the particle size, it may be due to the relatively high surface free energy of CaCO₃ nanoparticles and the strong distortion stress of crystal lattice.