In the present study, the antibacterial activity of the crude extracts obtained from flowers, leaves and stems of Hypericum calycinum L.against Streptococcus mitis CNCTC 4/77, Streptococcus salivarius CNCTC 64/59, Streptococcus mutans CNCTC 8/77 and Staphylococcus epidermidis ATCC 12228 was determined in vitro. Dried powder of flower, leave and stem parts of this plant were kept in acetone and methanol for 3 days in dark place. Disc diffusion method was used to determine the antibacterial activity of Hypericum calycinum L. Methanol-impregnated discs and acetone-impregnated discs were used as negative control. Also commercial antibiotic discs (Clindamycin (10 µg), Tetracycline (10 µg) and Amoxicillin-Clavulanic acid (30 µg)) were utilized as positive control. According to the results of the present study, the crude extracts of Hypericum calycinum L. prepared in acetone and methanol have been found to have antibacterial activity against the test microorganisms. Extracts of Hypericum calycinum L. flower showed the largest inhibition zone (13 mm in diameter) against S. mutans. The antibacterial activities of methanol and acetone extracts of Hypericum calycinum L.(at the doses of 3200 µg/10µl, 1600 µg/10µl, 800 µg/10µl and 400 µg/10µl)against Streptococcus salivarius, Streptococcus mutans, Staphylococcus epidermidis and Streptococcus mitis have been reported in this study for the first time