A biodegradable, immiscible poly(butylenes adipate-co-butylenes terephthalate) [P(BA-co-BT)]/poly(ethylene oxide) (PEO) polymer blend film with compositional gradient in the film-thickness direction has been successfully prepared in the presence of a low-molecular-weight compound 4,4'-thiodiphenal (TDP), which is used as a miscibility-enhancing agent. The miscibilities of the P(BA-co-BT)/PEO/TDP ternary blend films and the P(BA-co-BT)/PE0/TDP gradient film were investigated by differential scanning calorimetry (DSC). The compositional gradient structure of the P(BA-co-BT)/PEO/TDP (46/46/8 w/w/w) film has been confirmed by microscopic mapping measurement of Fourier-transform infrared spectra and dynamic mechanical thermal analysis. We have developed a new strategy for generating gradient-phase structures in immiscible polymer-blend systems by homogenization, i.e., adding a third agent that can enhance the miscibility of the two immiscible polymers through simultaneous formation of hydrogen bonds with two component polymers.