MicroRNAs (miRNAs) are a large family of post‑transcriptional regulators of gene expression that control a number of developmental and cellular processes in eukaryotic organisms and are ~23 nucleotides in length. miRNA‑122 is an abundant liver‑specific miRNA, implicated in fatty acid and cholesterol metabolism, as well as in hepatitis C viral replication and is frequently suppressed in primary hepatocellular carcinomas. In the current study, the Hep3B cell line with stable overexpression of miR‑122 was successfully established through gene transfection methods and drug screening. miR‑122 was observed to alter cell morphology in vitro by stable overexpression in Hep3B cells. This alteration was viewed by light microscopy and transmission electron microscopy. These alterations included increases in the cell volume, the appearance of lipid granules and vacuoles, thickening of nuclear membrane, swelling of the mitochondria, cytoplasm vacuolization and a more prominent nucleolus. Furthermore, the study provided novel evidence that miR‑122 function was dependent upon its expression level. In addition, it was observed to negatively regulate mitochondria.