The aim of this study was to evaluate in vitro the antibacterial effects of hydro-alcoholic extract of aerial parts of Origanum vulgare and nanoform Origanum vulgare extract on Streptococcus iniae, the causative agent of Streptococcosis in fish, in comparison with the effects of florfenicol and nonoform florfenicol on this bacteria. The results showed that the Minimum Inhibitory Concentration (MIC) and the Minimum Bactericidal Concentration (MBC) of Origanum vulgare extract was 1000 µg/ml whereas, the MIC and MBC of nanoform Origanum vulgare extract were 250 and 500 µg/ml, respectively. These results indicated that the inhibitory and bactericidal effects of Origanum vulgare extract was less than the nanoform Origanum vulgare extract. The MIC and MBC of florfenicol was 2.5 µg/ml while, the MIC and MBC of nanoform florfenicol were 0.625 and 1.25 µg/ml respectively. The results showed that the zones of inhibition for Origanum vulgare extract, nanoform Origanum vulgare extract, florfenicol and nanoform florfenicol were 21.70±0.6, 26.06±0.7, 31.63±0.4 and 32.80±1.1 mm, respectively. These results indicated that Origanum vulgare extract had antibacterial effects which were less than the effects of nanoform Origanum vulgare extract, florfenicol and nanoform florfenicol on Streptococcus iniae. In this study, the nanoform florfenicol was the most effective substance for controlling Streptococcus iniae infection.