Selective partitioning of fluorescein and zinc protoporphyrin into a liquid disordered (Ld) lipid domain was achieved in liposomes, providing an appropriate platform for fluorescence resonance energy transfer (FRET). The efficiency of FRET in the resulting liposome composites was regulated by simply adjusting the domain size. As the domain size was reduced, the density of fluorophores and zinc protoporphyrin in the Ld domain increased, resulting in the improvement of FRET efficiency.