The study of polaron dynamics in complex materials has garnered significant attention owing to its implications for various technological applications, including catalysis, solid-state devices, and energy storage. This paper investigates the photo-excited electron and hole polaron dynamics in cerium dioxide (CeO2) using time-resolved X-ray absorption spectroscopy, with an emphasis on the nano-size effect. Additionally, density functional theory and multiplet calculations have been utilized to reveal the photo-excited polaron dynamics in CeO2 single crystal (SC) and nanocrystal (NC). The electron polaron is observed to decay into a deep trap site with a short duration of ≈5 ps, while electrons in the traps stay for more than 1400 ps. The most significant observation is the behavior of holes in NC, which tends to stay longer (≈150 ps) compared to SC (