Aims To study the effects of diterpenoid pekinenal of Euphorbia pekinensis Rupr. on cell prolif-eration, cell cycle phase and apoptosis of hepatoma SMMC-7721 cells and to probe into its anti-cancer mechanisms. Methods MTT assay was used to inves-tigate the effect of pekinenal on proliferation of SMMC-7721 cells; TUNEL method, Annexin V/PI staining and electron microscopy were employed to observe the cell apoptosis; Flow cytometry was applied to analyze the distribution of cell cycle. Results Proliferation of SMMC-7721 cells was markedly inhibited by pekinenal in a dose-dependent manner; Annexin V/PI staining showed that with the increase of pekinenal concentra-tion, apoptotic rate of SMMC-7721 cells increased sig-nificantly, compared with control group, the difference has significant statistical significance ( P < 0. 01 ) . TUNEL method test results showed that different con-centrations of pekinenal in SMMC-7721 cells could in-duce liver cancer cell apoptotic and apoptotic index ( AI) increased significantly ( P <0. 01 ) with the in-crease of drug concentration. Compared with control group, electron microscope found that after the treat-ment, hepatocyte mitochondrial swelling, endoplasmic reticulum expansion, cytoplasm inclusion body forma-tion, part of the nucleus apoptosis, and the more obvi-ous apoptosis appeared with the increase of drug con-centration. Flow cytometry analysis showed that differ-ent concentrations of pekinenal could all make the cell block in S phase. Conclusion Pekinenal has an obvi-ously inhibitory effect on the human liver cancer cells, and there is significant concentration dependence; Pe-kinenal probably inhibits cancer cell DNA synthesis for the human liver cell cycle arrest in S phase and inhibits the proliferation . It plays a role in liver cancer inhibi-tion by inducing liver cancer cells apoptosis, etc.