Abstract: Carnosic acid (CA) is the main phenolic diterpenoid active ingredient in plants such as rosemary and sage, and has antiviral, antioxidant, anti-inflammatory effects and so on, however, its antiviral activity against influenza virus infections was not reported. In this study, antiviral activities against influenza A virus infections of three main bioactive ingredients from rosemary, including rosmarinic acid, CA and ursolic acid, were evaluated using virus titer titration assay, and CA showed remarkable inhibition on influenza H5N1 replication in A549 cells. The antiviral activity of CA was further confirmed and its mechanism of action was investigated using the indirect immunofluorescence assay (IFA), Western blot and real-time fluorescence quantification polymerase chain reaction (qRT-PCR). The results showed that the 50% effective concentration (EC₅₀) of CA against influenza H5N1 in A549 cells and MDCK cells were 4.30 and 3.64 μmol·L^-1, respectively. Meanwhile, CA also showed inhibition on influenza virus 2009panH1N1 (EC₅₀: 10.1 μmol·L^-1) and H3N2 (EC₅₀: 12.8 μmol·L^-1) replications in A549 cells. Mechanistic studies showed that antiviral activity of CA is related to its induction of heme oxygenase-1 (HO-1) in A549 cells and suppression on production of reactive oxygen in H5N1-infected cells.