The objective of the present study was to evaluate the in vitro antioxidant activity of the ethanolic extract of Limoniastrum guyonianum from Algeria. The plant was evaluated using the free radical scavenging activity of the 1.1-diphenyl-2-picrylhydrazyl radical (DPPH), reducing power assay and Phosphomolybdenum method. Total phenolic content (TPC) was measured using a Folin–Ciocalteu assay. Total flavonoid content (TFC) was measured by aluminum chloride colorimetric assay. Phenolic compounds was found in the L. guyonianum of El Oued region (LE)(south of Algeria) from (11.39 to 91.51 mg GAE/100 g DW) and L. guyonianum of Ouargla region (LO)(south of Algeria)from (15.51 to 75.81mg GAE/100 g DW). Effective scavenging Concentration (IC50) On DPPH radical ranged from (0.11 to 0.16 mg/l) in LO, and ranged from (0.18 to0.25 mg/l) in LE. The AEAC values of the L. guyonianum ranged from (1.8 to 2.16mM) in LO, and ranged from (0.55 to 2.14mM) in LE. The Phosphomolybdate antioxidant activity of the plant extracts ranged from (1.25 ± 0.07 to 7.94 ± 0.06 mM).