The essential oil extracted from Desfontaine's rupturewort, Herniaria fontanesii J. Gay subsp. fontanesii growing wildly in Tunisia, was analyzed using GC and GC-MS techniques. The free radical scavenging capacity and total phenol contents of three crude extracts having different polarities (n-hexane, ethyl acetate and methanol) were examined. Thus, a total of 35 constituents were identified in the Desfontaine's rupturewort essential oil representing 89.8% of the whole constituents. The oil was dominated by hexadecanoic acid, caryophyllene oxide, terpin-4-ol, khusimone and trans-sabinene hydrate. The total phenolic contents ranged from 16.91 to 92.27 mg of gallic acid/g of dry weight and they were found to be significantly higher in methanol than in polar ethyl acetate and hexane extracts. Correlations were observed between the phenolic contents and the antioxidant properties. Thus, the antioxidant activity of the methanol extract was superior to that of all samples tested (IC50 = 0.21 ± 0.04 mg/mL).