The plant growth and development are affected by oxidative stress under water deficit condition. The aim of this study was to investigate the effect of ascorbic acid as a strong antioxidant against oxidative stress caused by dehydration stress in olive trees cv. Mahali Baghmalek. In this study, two-year-old potted olive plants were exposed to three irrigation levels (100, 66 and 33% of plant evapotranspiration) and four levels of ascorbic acid (0, 250, 500 and 750 mg L^-1). Some biochemical parameters and enzymes activities were measured. This experiment was done as factorial experiment based on a randomized complete blocks design with three replications in 2015-2016. The highest chlorophyll a content (1.3 mg g^-1 FW) was related to trees treated with 250 mg L^-1 of ascorbic acid. Also, the highest amount of total soluble carbohydrates (9.5 mg g^-1 FW) and proline (2.17 µmol g^-1 FW) were obtained in plants irrigated with 33 and 66 % ET crop. Foliar application of ascorbic acid at 250 and 500 mg L^-1 resulted in the accumulation of total soluble carbohydrates and reduction in the activity of antioxidant enzymes such as catalase, peroxidase and oscorbat peroxidase. Therefore, according to the results of this experiment, it can be suggested that the use of ascorbic acid at a concentration of 250 and 500 mg L^-1 improved some biochemical properties and reduced the activity of antioxidant enzymes in olive cv. Mahali Baghmalek under water deficit stress (33 and 66% evapotranspiration).