The essential oil of the aerial parts of Pluchea dioscoridis (L.) DC, an endemic medicinal plant growing in Egypt, was obtained by hydrodistillation and analyzed by GC-MS. The TIC chromatogram of the oil exhibited more than 80 components; their percentages (as determined by normalization method) ranged from 0.1 – 18.3 %; fifteen of which are considered major (the percentages  of these components ranged from 1.34-18.3 %) and identified by library search of their MS data as well as by literature survey.  Our results are remarkably different from the previously reported oil constituents. Eight of the major components are reported for the first time in the oil, while four components have been previously reported in oil samples. In-vitro testing for its antioxidant activity by measuring the reducing power and DPPH free radical scavenging activity were determined to be 22.77 mg ascorbic acid equivalent (AAe)/mL of oil, and 1.65 mg AAe/mL of oil, respectively. The reducing power of the oil was compared with values determined for clove oil and eucalyptus oil.