Temperature stress is becoming the major concern for plant scientists worldwide due to climate change. Temperature stress has devastating effects on plant growth and metabolism. The aim of the study was to investigate the effect of climatic seasonal change on the yield and composition of essential oil of the plant, Achillea fragrantissima. The gas chromatography-mass spectrometry (GC-MS) used to analyze the essential oil collected during dry and wet seasons revealed it has 25 compounds. The major compounds were santolina alcohol (5.31), camphor (4.3), and cedrene (9.01) during winter months, while the percentages of α-cubebene (17.1%), spathulenol (1.54) and globulol (5.2) were highest during summer season. The analysis of essential oil in the two seasons revealed that there are different amounts and composition of essential oils. The antioxidant activity of the essential oil of the plant shows higher activity of IC 50 0.11±0.01g/L and Ec 50 0.25±0.02 g/L during winter than in summer. However, the reducing capacity of standard substances used (ascorbic acid and α-Tecopherol) were 0.033±0.001 and 0.93±0.07 g/L for DPPH and 0.091±0.002 and 0.026±0.002 g/L for FRAP method, respectively. These results showed that A. fragrantissima is a natural source of active compounds, and antioxidant properties, and the difference in chemical composition leads to changes in the antioxidant activity of the plant, which contributes to seasonal change.