RPB5-mediating protein (RMP) is associated with the RNA polymerase II subunit RPB5. RMP functionally counteracts the transcriptional activation of hepatitis B virus X protein that has been shown to play a role in the development of hepatocellular carcinoma (HCC). However, the effect of RMP on the growth of HCC remains unclear. In this study, we characterized the potential role of RMP in the proliferation of human HCC cells using two cell lines, SMMC-7721 and HepG2. We found that RMP expression increased when HCC cells were treated with (60)Co γ-irradiation. Cell growth and colony formation assays suggest that RMP plays an antiapoptotic role in the proliferation and growth of HCC cells. We also show that RMP depletion induced the G(2) arrest of HCC cells characterized by the decreased expression of Cdk1 and Cyclin B. Tumor formation assays further confirmed the in vivo requirement of RMP during HCC growth. In conclusion, our results demonstrate that RMP is a radiation-sensitive factor, and it may play essential roles in HCC growth by affecting the proliferation and apoptosis of HCC cells.