Photosynthetic organisms handle solar energy precisely to achieve efficient photochemical reactions. Because there are a wide variety of light-harvesting antennas in oxyphototrophs, the excitation energy transfer mechanisms are thought to differ significantly. In this study, we compared excitation energy dynamics between photosystem I (PSI) cores and a complex between PSI and fucoxanthin chlorophyll (Chl) a/ c-binding protein I (PSI-FCPI) isolated from a diatom, Chaetoceros gracilis, by means of picosecond time-resolved fluorescence analyses. Time-resolved spectra measured at 77 K clearly show that low-energy Chls in the FCPI transfer not only most of the excitation energy to the reaction center Chls in the PSI cores but also the remaining energy to carotenoids for quenching. Under room-temperature conditions, the energy in the low-energy Chls is rapidly equilibrated on Chls in the PSI cores by uphill energy transfer within a few tens of picoseconds. These findings provide solid evidence that the low-energy Chls in the FCPI contribute to the photochemical reactions in PSI.