Achillea millefolium is an important medicinal and aromatic plant and is grown commercially for production of essential oil in many countries including Egypt. The plant extract and essential oil possess antioxidant, antimicrobial, antispasmodic and antitumor properties. We hypothesized that the essential oil content and composition may be influenced by harvest time and environmental conditions. Thus, flowering heads yield and the essential oil content and quantitative composition were investigated during different harvest times in the Experimental Station of National Research Centre at Nubaria, Beheira Governorate, West of Nile Delta, Egypt under drip irrigation. Flowering heads yield and essential oil composition varied according to harvest time. The highest essential oil yield was observed at the second harvest on10 April and 5 May of the first and second seasons, respectively. The essential oils at the different harvest times were characterized by containing monoterpenes with high amounts of β-pinene (24.1–54.6%) and sabinene (3.1–7.4%). The most abundant sesquiterpenes were chamazulene (10.1–26.7%) followed by geramacrene-D (1.3–10.3%) and β-caryophyllene (0.9–6.4%). Plants harvested on 21 February provided essential oil with relatively high concentration of chamazulene and germacrene-D. Chamazulene and germacrene-D decreased gradually at the later harvests, while β-pinene and limonene reached the maximum concentration in plants harvested on15 May. These results indicate that flowering heads yield and essential oil content and composition can be influenced by harvest time and climatic conditions which can be useful to producers and processors of Achillea millefolium.