Validated by ab initio simulations on the spontaneous polarization, the molecular dynamics method is used to investigate the size effect on the polarization distribution, hysteresis behaviors and Curie temperatures of BaTiO3 nanowires. A quasi-axisymmetric radial distribution of polarization in the cross-section plane is disclosed. It is also found that there exist three typical polarization distribution scenarios for different wire diameters, and thus two critical diameters can be identified. Moreover, unusual stepwise hysteresis loops, and core-shell polarization structures with opposite polarization directions at the outer and inner regions of the nanowire are revealed in our simulations. This study on the atomic level polarization evolution may help us to understand the ferroelectric properties of nanowires.