In spite of great advances of modern scientific medicine, medicinal plants still make an important contribution to health care and they are form of treating diseases of majority of people in the world. The aim of this study was to evaluate the antimicrobial activity of extracts of Quercus robur L. (acorn) against some medically important microorganisms. Extracts of Q. robur L. (acorn) plant which was purchased from spice shop in the Eygptian Bazzar (Misir carsisi), Istanbul-Turkey, were prepared with methanol, ethanol, acetone, acetonitrile, petrolether, 0.25 M Tris HCl, chloroform, aqueous and were evaluated for activity against medically important microorganism such as Staphylococcus aureus ATCC 6538, Escherichia coli ATCC 25922, Enterococcus faecalis ATCC 29212, Pseudomonas aeroginosa ATCC 15442, Candida albicans ATCC 10231, Listeria monocytogenes ATCC 7644, Bacillus cereus RSKK 1122. The invitro antimicrobial activity was performed by agar well diffusion method and disc diffusion method. As a result of this study, it was found that methanol, ethanol, aqueous, acetone, acetonitrile, chloroform extracts showed antimicrobial activity at different levels while petrolether and 0.25 M Tris HCl did not exert any antimicrobial activity on tested microorganisms.