The medicinal plant yarrow (Achillea millefolium L.) belongs to the Asteracea family. It is being used in the pharmacological, food, and cosmetic industry. The economic importance of yarrow resides in its essential oil (EO). This plant is used in traditional medicine as the EO has properties which range from antibacterial, antifungal, anti-inflammatory, anti-oxidant and antitumor activities. The objective of this study was to identify chemical components and EO content of yarrow, as well as its antimicrobial activity against some micro-organisms in vitro. The fresh leaves were collected in a morning in October (2018) at the UNIPAR Medicinal Plants Garden, Umuarama-Paraná State, Brazil. The EO was obtained by hydrodistillation of the modified Clevenger type. After that, the content (m/m%) was calculated. The chemical composition of the EO was identified by gas chromatography/mass spectrometry (GC/MS). The minimal inhibitory concentration (MIC), minimal bactericidal concentration (MBC) and minimal fungicidal concentration (MFC) were determined by a microdilution method in 96-well microtitre plates and effect of EO was assessed on four micro-organisms (Candida albicans, Staphylococcus epidermidis, Escherechia coli and Klebsiella pneumoniae). The EO content (yield) in the plant shoots was 0.4% (four grams of EO kg-1 of plant fresh shoots) and presented 20 chemical compounds such as α-farnesene (31.66%), followed by chamazulene (17.17%), β-caryophyllene (10.27%) and sabinene (8.77%). The majority class was hydrocarbon sesquiterpene with 74.29%. The antimicrobial activity tests showed that the EO had low antimicrobial activity against the analyzed species with MIC for all species above 1.5 mg mL-1. It was concluded that the EO content was 0.4%. The major component was α-farnesene (31.66%) and EO presented low MIC.