This research studied the impact of planting location and cooking time periods of (5, 10, 15, 20 and 40 minutes) on the antioxidant effect, and on the total Phenols and Flavonoids contents using five different samples of Syrian onions. The results showed that the total Phenols, total Flavonoids and antioxidant activity contents were between (279-583)mg/100g dry wt, (80-180)mg/100g dry wt, and (60-75%) respectively. In addition, boiling for different periods resulted in increased total Phenols from (383 to789)mg/100g dry wt and in total Flavonoids from (124 to 205)mg/100g dry wt as well as an increase of (77% to 88%) of antioxidant activity as measured using the DPPH method.