Successful fennel (Foeniculum vulgare Mill) production in semi-arid environments depends largely on irrigation applications. Deficit irrigation studies on fennel are lacking or very limited, particularly in high-altitude environments. This study was conducted to determine the effects of deficit irrigation levels on the essential oil content, essential oil yield, and chemical composition of fennel. The experiment was carried out in 2014 and included four deficit irrigation levels: 100% (I-1), 80% (I-2), 60% (I-3), and 40% (I-4) of Class A pan evaporation. All parameters were significantly affected by deficit irrigation applications. Essential oil content tended to increase under increased water deficit conditions with the range of 2.21% to 2.42%. Anethole (90.71-91.62%) was the major compound in the fennel fruit oil, followed by estragole (3.60-4.02%), limonene (2.19-3.24%), and fenchone (0.96-1.55%), respectively. Essential oil yield showed an increase with increased irrigation amounts.