The genus Sium (Apiaceae) is presented by perennials with fibrous roots, inflorescence paniculate-corymbose, white petals, minutesepals, slightly lateral compressed fruits in the Flora of Turkey. The essential oils of the fruits and aerial parts of Sium sisarum L.collected from two different localities Kavak and Ladik in Samsun, Turkey, were obtained by hydrodistillation. The essential oilswere assigened as EO1 (fruit, Kavak), EO2 (aerial parts, Kavak), EO3 (fruit, Ladik), EO4 (aerial parts, Ladik). All samples wereanalyzed by gas chromatography (GC) and gas chromatography/mass spectrometry (GC/MS), simultaneously. γ-Terpinene (71.8,64.2, 42.3, 39.0 %) and p-cymene (16.4, 23.4, 33.8, 33.0 %) were found as major constituents of the analysed samples, respectively.β-Pinene (7.7 %) and limonene (8.3 %) were identified as the major components for EO3. Also, β-pinene (4.0 %), limonene (10.0 %)and ar-curcumene (2.0 %) were found as the major components for EO4. Additionally, the quantitative in vitro testing using DPPHwas conducted to determine the antioxidant capacity. UV absorbance of the samples were measured at 517 nm using a microplatespectrophotometer system. According to the assay results, antioxidant potential of the tested essential oils was comparativelyweak (IC50 > 30 mg/mL).