In the experiments, the radiosensitization of dihydrotanshinone Ⅰ on human hepatocelluar carcinoma cells QGY-7703 and lung adenocarcinoma cells SPC-A-1 was studied through in vitro colony formation method. The results showed that dihytanshinone Ⅰ has cytotoxity to the two kinds of cells and the LD,50, was 7.73,μ,mol/L and 8.86,μ,mol/L, respectively. The compound enhanced the radiation effect. Under aerobic condition the SER (sensitizing ehnancement ratio) of the compound was 1.42 for QGY-7703 cells and 1.62 for SPC-A-1 cells at the concentration of 1,μ,mol/L. The OER (Oxygen enhancemen tratio) for QGY-7703 cells and SPC-A-1 cells was 2.08 and 3.03,respectively. Under hypoxic condition dihydrotanshinone Ⅰ also had radiosensitivity for the two kinds of cells, and the SER of SPC-A-1 cells increased to 1.96 while the SER of QGY-7703 kept 1.42. In split-dose radiation experiment,dihydrotanshinone Ⅰ could inhibit SLDR (sublethal damage repair) obviously at the concentration of 1,μ,mol/L.