When BaTiO3 thin films are deposited on SrTiO3 (001) substrates at low temperatures at approximately 700 degreesC, at first epitaxial growth without defects occurs, but at a critical thickness it changes into pseudomorphic growth by introducing misfit dislocations. Cross-sectional high-resolution transmission-electron microscopy shows that these dislocations lie at the interface and stacking faults on (101) planes are also observed. Molecular dynamics simulations were performed in order to understand the formation of these dislocations. The critical thickness for occurrence of misfit dislocations is found to be approximately four monolayers with a slight increase with temperature. The misfit dislocations dissociate into partial dislocations and migrate to the interface by forming stacking faults on (101) planes.