ABSTRACT The objectives of this study were to determine the protective effect of quercetin on NSAIDs-induced gastric mucosal damage by measuring the level of superoxide radical, hydrogen peroxide (H2O2), protein carbonylation (PC), and advanced oxidation protein products (AOPPs). In this experiment, a gastric mucosa was taken from male rats (Rattus novergicus). Samples then homogenized and divided into three groups with; T1 served as control which contains gastric mucosa homogenate only; T2 which contains gastric mucosa homogenate + 500 mg of mefenamic acid; and T3 which contains gastric mucosa homogenate + 500 mg of mefenamic acid + 250 mg/L of quercetin. After treatment, every 15- minute superoxide radical, H2O2, PC, and AOPPs levels were estimated. The results revealed that mefenamic acid increased the rate of superoxide radical, H2O2, PC, and AOPPs formation, while quercetin decreases the rate of all parameters. These results indicated that mefenamic acid induce gastric mucosal damage through oxidative stress and inflammation mechanisms as can be seen from the increasing of superoxide radical, H2O2, PC, and AOPPs levels. Also, the results indicated that quercetin have a protective effect to gastric mucosa as can be seen from the decreasing of all parameters. Keywords: Gastric Mucosal Damage, Inflammation, Mefenamic Acid, Oxidative Stress, Quercetin