The presence of 180° ferroelectric domains in polycrystallineBaTiO3thin films has been observed with the aid of transmission electron microscopy. The 180° domain boundary exhibited a curved appearance, and its orientational relationship was demonstrated. It was found that equiaxial grains did not have 180° domain boundaries due to their higher boundary energy. The grain size effect on dielectric constant of polycrystallineBaTiO3thin films was also discussed in conjunction with 180° ferroelectric domains. ©1998 American Institute of Physics.