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Aims: This work aims to study the effect of the harvest time (pre-flowering and full-flowering) on the yield, chemical composition, antimicrobial and antioxidant activities of Thymus vulgaris and Mentha pulegium essential oils (EOs). Study Design: Leaves and flowers from each period (pre and full-flowering) were used for EOs extraction. EO yield, chemical composition, antimicrobial and antioxidant activities were determined for each plant at the two harvest times studied. Place and Duration of Study: The studied plants (T. vulgaris and M. pulegium) were collected during the period of April 2011 (Pre-flowering) to June 2011 (Full-flowering). Experiments were conducted at the chemistry and microbiology laboratory of the National Institute of Agronomic Research of Tangier (Morocco). Methodology: The EOs were extracted via steam distillation. Chemical composition has been determined by a GC/MS analysis. Antimicrobial activity against Escherichia coli (SCTC 471), Salmonella Senftenberg 775W (ATCC 43845), Listeria monocytogenes (SCTC 4031) and Staphylococcus aureus (SCTC 976) was determined by paper disc agar plates. Antioxidant activity were determined by the radical scavenging activity assay. Results:The greatest yield for T. vulgaris (3.6%) and for M. pulegium (3.5%) EOs was obtained during the full-flowering period. The chromatographic analysis showed that the studied EOs were constituted mainly by carvacrol for T. vulgaris and pulegone for M. pulegium. Harvest time affected quantitatively but not qualitatively the chemical composition of both EOs. T. vulgaris EO showed a greater antimicrobial and antioxidant activity than that of M. pulegium. The antimicrobial and antioxidant activities were maximal during the full-flowering period for T. vulgaris EO whereas they were greater in the pre-flowering period for M. pulegium EO. Conclusion:The full-flowering period would be the best time to harvest T. vulgaris plants to obtain EOs with better yield, antimicrobial and antioxidant activity. In contrast, the pre-flowering stage would allow producers to obtain a M. pulegium EO with higher antimicrobial and antioxidant activities although with a lower yield.