Abstract: Ursolic acid(UA),a pentacyclic triterpenoid compound,is widely distributed in the plant kingdom.The present work was performed to investigate the potential role of UA in the differentiation of human monocytic leukemia cell line THP-1 as well as the underlying mechanisms for it. After 72 h treatment with UA, morphology of THP-1 cells was observed by phase contrast microscope. Some of the cells were adherent and turned into macrophage-like cells. Moreover, the adhesion rate was enhanced in a dose-dependent manner,detected by crystal violet staining assay. In addition, the differentiation markers,CD11b and CD14,the positive cells were significantly increased after the treatment with UA analyzed by flow cytometry. Finally, western blotting showed that UA up-regulated p42 C/EBPα protein, while inhibiting the expression of c-myc, both in a concentration- and time-dependent manner. Taken together, ursolic acid can induce THP-1 cells differentiation into monocyte/macrophage lineage probably through the regulation of p42 C/EBPα and c-myc.