Allium cepa L. belonging to Amaryllidaceae, is considered to be one of the most cultivated and consumed vegetable in the world. Besides nutritional value, as medicinal usage has also been reported. Onion has been cultivated widely and used as a vegetable mainly, in Turkey. Treatment of wounds, abscess, hemorrhoids, arteriosclerosis, gynecological problems, urinary inflammation, headache, cough by usage of onion and its diuretic and hypoglycemic activities are recorded in Turkish folk medicine. In this study, antimicrobial, antioxidant, anti-inflammatory activities and total phenolic-flavonoid contents of red, white and yellow onion samples from Ankara-Turkey, were investigated. The peels and tissues of onion samples were extracted by sonication with 80% ethanol at room temperature. Phytochemical contents were investigated by High Performance Liquid Chromatography. The highest amount of quercetin was detected in red onion peel (0.3169 mg/100 mg); and protocatechuic acid in yellow onion peel (0.3492 mg/100 mg). Determining the antimicrobial activities against Staphylococcus aureus, Enterococcus faecalis, Escherichia coli, Pseudomonas aeruginosa by using microbroth dilution method, peel extracts exhibited higher activity than tissues. Yellow onion peel displayed the best 2,2’-azino bis (3-ethylbenzothiazoline-6-sulphonic acid) and 2,2-diphenyl-l-picrylhydrazyl scavenging activity (IC50=0.0125 and 0.0433 mg/ml, respectively) and also human red blood cell membrane stabilization activity which was used as a marker of anti-inflammatory activity (IC50=0.5363 mg/ml). Supporting these biological activities, the highest flavonoid content was determined in yellow onion peel (186.9250 mg QE/g crude extract). Additionally, red onion peel displayed the highest superoxide anion radical scavenging activity (IC50=0.0987 mg/ml). All results have revealed that peel extracts possessed higher activity than tissues.