Although barium titanate ( BaTiO ₃) ferroelectrics at the nanoscale has been extensively investigated, the physical origin of their unusual performance, in particular, the suppressed thermal stability remains yet unclear despite existing models from various perspectives. Based on the consideration of the short range bond order-length-strength correlation and the long range dipole-dipole interaction, we have developed a dual-shell model for the size and shape induced suppression of the Curie temperature for BaTiO ₃ nanocrystals. One surface shell of three atomic layers represents the short range interaction due to the shorter and stronger bonds between under-coordinated atoms, which intrinsically lowers the mean atomic cohesive energy of the crystal. The other shell with thickness of K_c (critical number of atomic layers) characterizes the long rang dipole-dipole interaction. If one moves a BaTiO ₃ unit cell from the center of the nanocrystal outwards, the unit cell will lose its ferroelectrics gradually upon reaching the K_c point. The modeling predictions have been verified by the presented experimental observations and results documented in the open literature. The least K_c has been optimized to be 9 for BaTiO ₃ spherical particles.