We describe numerical simulations of fluorescence resonance energy transfer (FRET) in symmetric diblock copolymer lamellae. The model systems were generated by Monte Carlo simulation at various finite temperatures, and virtual dyes (donors (D) and acceptors (A)) were introduced at the junction points of the block copolymer chains. We examined the effect of dye dipole orientation on energy transfer by selectively choosing the dipole orientations. The coupling effect between dipole orientation and D-A distance in the calculation of extent of energy transfer was found to be significant when donors and acceptors were confined to a thin interface at a low simulation temperature. We calculated the interface thickness of the simulated diblock copolymer lamellae based on the FRET data and the Helfand-Tagami mean field theory of polymer interfaces. FRET operates over short distances and is insensitive to the waviness of the interface.