Epitaxial PbTiO3 and BaTiO3 thin films were pl spared on MgO(100) by laser ablation and RF magnetron sputter deposition, respectively. X-ray diffraction (XRD), transmission electron microscopy (TEM), and atomic force microscopy (AFM) were used to investigate the ferroelectric domains in the grown films. in the PbTiO3 film, significant portions of a-domains were distributed in c-domains, which causes abundant 90 degrees domain population, On the other hand, the BaTiO3 film was mainly single c-domains without any meaningful domain structure. We attribute the difference in domain population of the two films to two reasons. One is the smaller phase transformation strain which comes from smaller tetragonality of BaTiO3, and the other is the larger compressive stress due to thermal mismatch which comes from lower T-c, of BaTiO3 than those of PbTiO3.