Abstract: Currently, improving the oral bioavailability of poorly soluble drugs is a hot topic in pharmaceutics, with particularly wide attention to nanocrystallization due to its high drug loading, high oral bioavailability and easy industrial production. Drug nanocrystal refers to nanoscale particles prepared by pulverization (Top-down method) or nanoprecipitation (Bottom-up method) with no other pharmaceutical carriers except stabilizer. It is generally believed that nanocrystals are absorbed into blood through endocytosis or transported to the mesenteric lymphatic circulation mediated by M cells after oral administration, but the trans-epithelial mechanism of nanocrystals has not been thoroughly explored. This article systematically introduces the definition of nanocrystals, their preparation methods, some in-vitro cell culture models (Caco-2, MDCK, Caco-2/HT29-MTX, Caco-2/HT29-MTX/Raji-B) and influential factors for the trans-epithelial mechanism of nanocrystals, and analyzes the trans-epithelial mechanism of nanocrystals at molecular level through in-vitro cell culture models.