Nanosized molecules possess enormous potential in imaging and therapeutic treatment. Due to their special physiochemical properties, their in-vivo behavior including absorption and biodistribution is worth to be studied. Zebrafish have emerged as useful vertebrate models for the optical tracking and biocompatibility assessment because of their transparent body and similar homology to human. This study aimed to explore the absorption and biodistribution of nanocrystals in zebrafish model. A highly lipophilic compound named coumarin-6 (C6) was selected and formulated as C6-nanocrystals (C6-NCs) by anti-solvent precipitation method with an average particle size of 67.5 ± 5.2 nm. Larval zebrafish at 8 days post-fertilization were incubated with different concentrations of C6-NCs.Whole body of zebrafish was collected from 5 to 60 min to take fluorescence images. C6 was also extracted and quantified by florescence detector. As shown in Figure 1, A, no obvious fluorescence was observed in C6 suspension treated group, while strong fluorescence was observed in C6-NCs treated group, and the intensity of fluorescence emission increased at dose dependent manner. Above result was consistent with the quantification result shown in Figure 1, B. The concentration of C6 increased from 5 min to 30 min and reached to a plateau at 60 min (Figure 1, C). After immersing for 30 min, C6-NCs were replaced by fresh medium. Zebrafish were collected and processed at different times up to 48 h. As shown in Figure 1, D, C6-NCs were significantly uptaken by zebrafish with clear regions of strong fluorescence in the eye, gall bladder, intestine and brain. C6-NCs were readily absorbed and distributed in zebrafish but eliminated very slowly. After 48 h, there was still a part of C6 accumulated in the zebrafish body, especially in the digestive system. These findings suggest that nanocrystals could enhance in-vivo absorption and biodistribution of C6 compared to the free drug. In conclusion, zebrafish could be developed as a useful in-vivo model for preliminary and rapid evaluation the absorption and biodistribution of the nanosized formulation.