Radiotherapy plays an important role in the treatment of hepatocellular carcinoma (HCC). Cyclin G1 is a novel member of the cyclin family, and it is abnormally expressed in HCC. In this study we investigated the role of cyclin G1 in the radiotherapy of HCC cells. The expression of cyclin G1 was silenced by transfection of cyclin G1-siRNA into HepG2 cells and Huh7 cells, and the expression of cyclin G1 mRNA and protein was measured by qRT-PCR and Western blot analysis. The proliferation was analyzed using MTT assay, and the radiosensitivity of HCC cells was detected using colony formation assay and a xenograft tumor model. The expression of apoptosis-related proteins (Bcl-2 and Bax) was detected by Western blot analysis, and caspase-3 was detected using fluorimetry. The expression of cyclin G1 mRNA and protein in HepG2/Huh7-cyclin G1-siRNA cells was found to be significantly decreased compared to that in HepG2/Huh7 cells. Silencing the expression of cyclin G1 inhibited the proliferation of HCC cells and enhanced radiosensitivity in HCC cells in vitro and in vivo. Knockdown of cyclin G1 expression significantly decreased Bcl-2 expression, and increased Bax expression and caspase-3 activity in HCC cells. Silencing of cyclin G1 expression enhances the radiosensitivity of HCC cells in vitro and in vivo. The mechanism for this may be related to the regulation of apoptosis-related proteins.