After the discovery of Fe-based superconductors, many groups have worked on preparing epitaxial thin films to explore their electro-magnetic and optical properties as well as possible applications. To date, epitaxial Fe(Se,Te), hole or electron doped AEFe₂As₂ (AE: alkari earth elements, AE-122), and LnFeAsO (Ln: rare earth elements, Ln-1111) thin films have been prepared by pulsed laser deposition (PLD), RF sputtering and molecular beam epitaxy. We have fabricated Co-doped Ba-122 thin films on various single crystalline substrates by PLD and investigated the effect of epitaxial strain on the superconducting properties. In this study, we found that the lattice distortion clearly affects the transition temperature, T_c. This was further confirmed by the experiments performed on piezoelectric substrates, in which the in-plane lattice parameter can be controlled by electric field. Later, the presence of a thin epitaxial Fe at film / substrate [(La_0.7Sr_0.3)(Al_0.65Ta_0.35)O₃] interface was confirmed by the detailed microstructural analyses. Additionally, we have found that a bcc Fe bonds coherently to FeAs tetrahedron in the Co-doped Ba-122. Since FeAs or FeAs(Te) tetrahedron is a common structure for Fe-based superconductors, the implementation of an Fe buffer may be applicable to their epitaxial growth. Indeed, epitaxial Fe(Se,Te)/Fe bilayers with sharp out-of- and in-plane texture have been realised. Fe-buffer engineering also offers unique phenomena, namely strain induced superconductivity in parent Ba-122 compounds.