a-/b-axis-oriented epitaxial Bi4Ti3O12 and neodymium-substituted Bi4Ti3O12 films with a different a-domain fraction, V(100)/[V(100)+V(010)], were grown by metalorganic chemical vapor deposition above the phase transition temperature. It was demonstrated that the saturation polarization observed for the a-/b-axis-oriented film is proportional to the a-domain fraction estimated by x-ray diffraction. The liner relationship passing through the origin revealed that the 90° domain switching by an external electric field hardly occurred. The extrapolation gave spontaneous polarization of 58 μC/cm2 for a pure a-axis-oriented (Bi3.5Nd0.5)Ti3O12 film. The domain fraction was investigated as a function of thermal strain originated from a difference in thermal expansion coefficient between the film and substrates. The domain fraction of the films changed with the thermal strain along the in-plane  in tetragonal a-axis-oriented films as well as epitaxially grown tetragonal Pb(Zr,Ti)O3 films.