This study was conducted to determine in vitro the antioxidant and antimicrobial potency of essential oils from two medicinal plants known in the Algerian northwest (Mascara city): Atriplex halimus L. and Centaurium erythraea Rafn. The analysis of essential oils (EOs) chemical compounds was performed by GC/MS. In total, 72 and 35 chemical components were identified for C. erythraea and A. halimus, which represents respectively 91.89% and 89.17% of the essential oil content. In fact, EO of A. halimus abundantly contained viridiflorol (40.23%), phytol (18.24%), germacrene D (6.94%), whereas β-copaen-4α-ol (38.41%), manool (8.2%) and carvacrol (6.43%) were found in OE of C. erythraea. Both essential oils were tested for antimicrobial activity against Staphylococcus aureus, Enterococcus faecalis (Gram-positive bacteria), Escherichia coli, Pseudomonas aeruginosa (Gram-negative bacteria) and one yeast strain Candida albicans using the agar-disc diffusion assay and the microdilution method (Minimum Inhibitory Concentration, MIC). A. halimus EO is active against Escherichia coli, whereas the essential oil of C. erythraea is active against Staphylococcus aureus, Escherichia coli and Candida albicans. The antioxidant properties were evaluated using free radical scavenging and ferric reducing power (FRAP) assay. The results obtained showed the existence of an antioxidant activity of the studied essential oils but less effective compared to the standards used (ascorbic acid and catechin).