We report on the controlled multiphase thin film growth in the Bi–Fe–O system. By varying the deposition oxygen pressure, the dominant phase formed in the film continuously changes from ferroelectric BiFeO3 to a mixture of α-Fe2O3 and ferromagnetic γ-Fe2O3. X-ray diffraction and high-resolution transmission electron microscopy have revealed that epitaxial multiferroic nanocomposites consisting of BiFeO3 and Fe2O3 are formed when the deposition pressure is ≈5 mTorr. In order to investigate the previously reported anomalous enhancement in magnetization in BiFeO3, we have fabricated a thickness gradient pure BiFeO3 film. The out-of-plane lattice constant was found to increase continuously as the thickness is decreased from 300 to 5 nm, but no significant enhancement in magnetization was observed.