Combining radiosensitizers with ionizing radiation (IR) is an effective strategy to increase the radiation therapeutic effect for hepatocellular carcinoma (HCC) patients. A phenanthroline derivative, 2-phenyl-imidazo [4, 5 f] [1, 10] phenanthroline (L02), had been synthesized. This study investigated the radiosensitization and mechanisms of L02 combined with IR against HCC. The radiosensitization of L02 combined with IR was evaluated by the sensitivity enhancement ratio (SER) and the isobolographic analysis. The toxicity of L02 and cisplatin were compared by the zebrafish model. The cell cycle and apoptosis were examined by flow cytometry. DNA damage was measured by comet assay and the expressions of apoptosis related proteins were analyzed by western blotting. L02 was effective in sensitizing HCC to IR. The SERs in HepG2 and BEL7402 were 1.41 and 1.28, respectively. The sensitization of L02 was comparable with cisplatin. L02 treatment with IR had synergistic anti-tumor effect. L02 enhanced the percentage of IR induced apoptosis cells. L02 increased comet tail in comet assay when combined with IR. L02 sensitized HCC to IR by the activation of P53 signaling, the decrease in Bcl-2, up-regulation of cytochrome c and the subsequent activation of caspase-3. L02 sensitizes HCC to IR, mostly likely by inhibiting cell proliferation, inducing DNA damage and mitochondria-dependent apoptosis. L02 may be a novel radiosensitizer for HCC.