Here, we report that nanodiamonds (NDs) exhibit blue fluorescence with an emission peak at around 400 nm. With increasing temperature, the peak energy of fluorescence was found to demonstrate a blue shift, possibly due to excited excitons populating higher-energy states, such as oxidation defect states. The intensity evolution of the fluorescence was attributed to a thermally activated process. Moreover, the bandwidth of fluorescence also increased because of exciton–phonon interactions and ionized impurity scattering. The above results indicate that the fluorescence of NDs could originate from radiative recombination through intrinsic transitions between highly localized π states.