The clinical development of natural product tanshinone I (1) for cancer therapy is hampered by its weak potency and poor drug-like properties. Herein, a more broad and systemic structural modification on 1 was conducted to generate four series of new tanshinone derivatives. Among them, the lactam derivative 22h demonstrated the most potent antiproliferative activity against KB and drug-resistant KB/VCR cancer cells, which are approximately 13- to 49-fold more potent than 1. Compound 22h possesses significantly improved drug-like properties including aqueous solubility (15.7 mg/mL), metabolic stability of liver microsomes, and PK characters (T1/2 = 2.58 h; F = 21%) when compared to 1. Preliminary mechanism studies showed that 22h significantly induced apoptosis of HCT116 cells, at least partially, through activation of caspase-3/-7. More importantly, administration of 22h at 10 mg/kg significantly suppressed the tumor growth of HCT116 xenograft in vivo without significant loss of body weight of the tested nude mice.