The aim of the present study was to investigate the role of microRNA (miR)‑21 in regulating collagen I and Smad7 expression in activated rat hepatic stellate cells (HSCs). Rat HSCs were isolated by single‑step density gradient centrifugation with Nycodenz. Cellular content of miR‑21, SMAD7, α‑smooth muscle actin (α‑SMA), collagen type I alpha 1 (COLLA1) and COLL alpha 2 (A2) mRNA was examined by reverse transcription‑quantitative polymerase chain reaction (RT‑qPCR), and cellular content of Smad7 and α‑SMA protein was detected by western blotting. Binding of miR‑21 to the 3'‑untranslated region (UTR) of Smad7 was verified by dual‑luciferase assay. The authors observed that, in activated HSCs, expression of miR‑21 was significantly increased in a time‑dependent manner, while expression of Smad7 mRNA and protein was significantly reduced. In addition, miR‑21 mimics significantly enhanced cellular α‑SMA mRNA and protein content, while miR‑21 inhibitor significantly reduced α‑SMA mRNA and protein levels. Similarly, cellular content of COLLA1 and COLLA2 mRNA was significantly elevated by miR‑21 mimics, but reduced by miR‑21 inhibitor, in activated HSCs. Moreover, cellular content of Smad7 mRNA and protein was significantly reduced by miR‑21 mimics, but significantly increased by miR‑21 inhibitor. Furthermore, miR‑21 mimics activated firefly luciferase in HEK293 cells transfected with the wild type 3'‑UTR of Smad7. miR‑21 regulates expression of α‑SMA and collagen I in activated rat HSCs by directly targeting Smad7.