ABSTRACTEffects of defect on ferroelectric stability in PbTiO3 (PTO) thin films have been investigated using molecular dynamics with first-principles effective Hamiltonian. In this paper, oxygen vacancy (Vo) has been considered to study the hysteresis loop, spontaneous polarization as a function of film thickness. Vo has been modeled as a charged point defect. Density functional theory (DFT) calculations are performed to determine the Vo-induced localized fields (both mechanical and electrical) and the calculated DFT results are used as inputs to molecular dynamics simulations in a large system. The strain field induced by the Vo is calculated by DFT calculations and fitted by the continuum strain modeling, and the electrostatic field is given by the superposition of the Vo-induced field and the external field. Vo significantly reduces the spontaneous polarization and increases the critical thickness.