The changes in total phenolic content (TPC), total flavonoid content (TFC), proline, malondialdehyde (MDA), H2O2, and antioxidant activity were assessed based on three model systems in three Achillea species (Achillea millefolium, A. nobilis, and A. filipendulina) growing under four irrigation regimes, including 100% FC (field capacity as normal irrigation) 75% FC (low stress), 50% FC (moderate stress), and 25% FC (severe stress) conditions. The highest TPC (47.13 mg tannic acid/g DW) and TFC (20.86 mg quercetin/g W) were obtained in A. filipendulina under moderate and severe stress conditions. In 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay, the highest and the lowest antioxidant activity was obtained for A. millefolium (70.28%) and A. filipendulina (53.21%), respectively, while in the FTC model system A. nobilis revealed the highest antioxidant activity (1.934) in severe drought condition. In the linoleic model system, the highest antioxidant activity was observed under low drought stress condition in A. nobilis. MDA and H2O2 content were increased due to both low (75% FC) and moderate (50% FC) drought stress, but they were decreased under severe stress condition (25% FC). Furthermore, A. millefolium revealed the lowest H2O2 (4.96 nm/g FW) and MDA content (176.32 μmol/g). Investigation of the relationship among different metabolites showed a strong positive correlation with TPC and TFC. Finally, the moderate drought stress treatment (50% FC) was introduced as the optimum condition to obtain appreciable TPC and TFC,, while the highest antioxidant activity was obtained in severe stress condition (25%FC).