In this paper, cellular management of fluorescent nanodiamonds (FNDs) has been studied for better understanding in the design for potential applications of FNDs in biomedicine. The FNDs have shown to be photostable probes for bioimaging and thus are well-suited, for example, long-term tracking purposes. The FNDs also exhibit good biocompatibility and, in general, low toxicity for cell labeling. To demonstrate the underlying mechanism of cells coping the low but potentially toxic effects by nondegradable FNDs, we have studied their temporal intracellular trafficking. The FNDs were observed to be localized as distinct populations inside cells in early endosomes, lysosomes, and in proximity to the plasma membrane. The localization of FNDs in early endosomes suggests the internalization of FNDs, and lysosomal localization, in turn, can be interpreted as a prestate for exocytosis via lysosomal degradation pathway. The endocytosis and exocytosis appear to be occurring simultaneously in our observations. The mechanism of continuous endocytosis and exocytosis of FNDs could be necessary for cells to maintain normal proliferation. Furthermore, 120 h cell growth assay was performed to verify the long-term biocompatibility of FNDs for cellular studies.