Starting from commercial available natural product oridonin (1), a practical synthesis of ent-6,7-seco-oridonin derivatives (2, 3, 5, and 9) was accomplished and their biological activities were evaluated. The conversion of spirolactone-type diterpenoid to enmein-type was first completed. The results demonstrated that all synthesized ent-6,7-seco-oridonin derivatives could markedly inhibit the proliferation of cancer cells. Compared with Taxol, the most cytotoxic compound 5 has similar potency in A549 cell and slightly less cytotoxicity in Bel-7402 cell. Compound 5 was also more potent than parent compound oridonin in mice with MGC-803 gastric cancer in vivo. Then a series of novel 14-O-derivatives of 5 were further designed and synthesized, which showed better activity than 5 and similar activity as Taxol in vitro. The structure-activity relationships of oridonin derivatives were also discussed in the present investigations.