Abstract: Objective To investigate the effects of cyclooxygenase inhibitor diclofenac on the proliferation and cyclooxygenase-2 (COX-2) mRNA expression of cultured hepatocellular carcinoma cell lines HepG2, Hep3B and human hepatocellular cell line QSG-7701. Methods After exposure to diclofenac at various concentrations (10-200 μmol/L) for 24, 48 and 72 h, the cell proliferation was analyzed by Cell Counting Kit-8 (CCK-8) assay and mRNA expression determined by semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR). Results Diclofenac exposure for 24, 48 and 72 h significantly inhibited HepG2 and Hep3B cell proliferation in a concentration-dependent manner, with inhibition rate of 40.47% and 54.49% after 48 h exposure to 50 μmol/L diclofenac and IC50 of 70.54 and 48.39 μmol/L, respectively. A much weaker antiproliferative effect on QSG-7701 cells was shown, with IC50 of 189.91 μmol/L after 48-hour exposure to diclofenac. RT-PCR detected COX-2 mRNA in HepG2 and Hep3B cells, but hardly in QSG-7701 cells. Treatment with diclofenac or 5-Fu resulted in elevated COX-2 mRNA expression both in HepG2 and Hep3B cells. Conclusions Diclofenac can specifically inhibit the proliferation of COX-2-expressing HepG2 and Hep3B cells, and induce up-regulation of COX-2 mRNA expression, which indicates the important role of COX-2 in the proliferation of hepatoma cells.