An inter-component epitaxial strain-induced PbTiO3 metastable phase is observed in a PbTiO3-CoFe2O4 epitaxial composite film,corresponding to the dielectric anomaly reported previously.High-resolution synchrotron radiation X-ray diffraction and first principles calculation demonstrate the coexistence of different Pb TiO3 phases,even a possible morphotropic phase boundary in the film,elucidating the underlying microscopic mechanism of the formation of Pb TiO3 metastable phase.This sheds light on the design and manipulation of electromechanical properties of epitaxial films,through the strain engineering.