Abstract: The morphologies and interface structure of self-assembled 0.65BiFeO₃-O.35COFe₂O₄ nanocomposite thin films on (001)-SrTiO₃ single crystal substrates were studied by advanced transmission electron microscopy (TEM) techniques at the atomic scale. During the process of thin-film epitaxial growth, BiFeO₃ (BFO) and CoFe₂0₄ (CFO) separate spontaneously and form a self-assembled composite nanostructure. The ferromagnetic spinel CFO forms nanopillars presenting a rectangular shape embedded in the ferroelectric perovskite BFO matrix, and both grow epitaxially along the[001] orientation and then get a vertical columnar nanostructure. The two phases give a cube-on-cube orientation relationship,[001]BFO//[001]CFO and (100)BFO//(100)CFO, and the interface between them lies in the {110} orientation. The surface of the composite film is rough with {111} facets of CFO and a smooth surface of BFO resides on the (001) plane. Energy-dispersive spectroscopy (EDS) studies show that the constituents of each phase present homogeneous distribution and no obvious interdiffusion is observed across the interface.