Hedyotis diffusa Willd. (Rubiaceae) is an important folk herb used to prevent and cure hepatitis and liver cancer in Taiwan. For differentiation of H. diffusa from counterfeits, macroscopic and microscopic characters of H. diffusa, H. corymbosa and H. tenelliflora were examined in this study. According to Trypan blue exclusion assay and Western blot analysis, H. diffusa had a significant inhibition of cell growth and induction of cell apoptosis in COLO 205 (colon cancer), Hep 3B (hepatocellular carcinoma) and H460 (lung cancer) cell lines. This study also used high-performance liquid chromatography (HPLC) to determine the quality control of H. diffusa. The HPLC data showed that ursolic and oleanolic acid are the components of the H. diffusa, consisting of approximately 4.66-4.80% and 1.86-1.96%, respectively. Our study also demonstrated that ursolic acid has significant anti-tumor activity in COLO 205, Hep 3B and H460 cancer cells.