Omega-3 fatty acids are often attempted to incorporate into foods due to their beneficial effects on health. However, their susceptibility to autoxidation limits their utilization. After analyses, rosemary extracts (REs) contained abundant antioxidant phenolics, flavonoids, and condensed tannin. Therefore, the aims of this study were to (1) determine the optimal extraction condition of a natural antioxidant from rosemary leaves; (2) investigate an lipid stability of plant-derived omega-3 fatty-acid rich oil (flaxseed oil) with an RE addition. Rosemary extracts obtained by using 80% ethanol solution extraction for 30 min had higher (p < 0.05) phenolic recoveries and better (p < 0.05) in vitro antioxidant abilities. Furthermore, REs can retard (p < 0.05) lipid oxidation of flaxseed oil under a Schaal oven test condition (60 °C) compared to α-tocopherol (α-TOCO) and butylated hydroxytoluene (BHT). In conclusion, REs have a positive efficacy to stabilize flaxseed oil against oxidation, posing as a potential application of omega-3 fatty acids in foods.