miR-122 is highly abundant in liver and a hepato-specific microRNA. There is evidence to show that miR-122 expression is down-regulated in human hepatocellular carcinoma (HCC). It is not known whether miR-122 affects the cellular behavior of hepatoma cells. The aim of this study was to investigate the effects of miR-122 on the viability and apoptosis of hepatoma cells. The viability and apoptosis of Huh-7 and HepG2 cells treated with miR-122 or miR-122 antisense RNA (anti-miR-122) were analyzed by adenosine triphosphate (ATP)-based luminescent assay, annexin V-based flow cytometry, and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) detection. The miR-122 coding genes in both cell lines were sequenced. Although two putative promoter sequences for the miR-122 gene at 18q21.31 were detected, the miR-122 coding sequence was missing in HepG2 cells, which might be the reason for the absence of miR-122 expression. There was no significant difference between the viabilities of HepG2 cells transfected with miR-122 and mock HepG2 cells (p >0.05). However, the viability of Huh-7 transfected with anti-miR-122 was significantly elevated at 24, 36, and 48 h posttransfection compared with that of mock cells (p <0.01). Both the flow cytometry and TUNEL assay showed that the apoptotic level of Huh-7 transfected with anti-miR-122 was significantly decreased at 48 h posttransfection (p <0.01). miR-122 down-regulated the viability but up-regulated the apoptosis of hepatoma cell Huh-7. The absence of miR-122 expression in HepG2 cells was due to the loss of the miR-122 coding sequence in chromosome 18. These results imply that aberrant expression of miR-122 may contribute to hepatocarcinogenesis.