Antioxidant activity of 70% aqueous ethanolic extract of leaves of Justicia gendarussa (EJ) was evaluated. EJ was prepared by cold maceration method. The antioxidant potency of EJ was investigated employing various established in vitro systems, such as DPPH radical scavenging, nitric oxide (NO) scavenging, ${eta}-carotene$ linoleic acid module system (${eta}$ CLAMS), hydroxyl (OH) radical scavenging, anti lipid peroxidation. $IC_{50}$ values were determined in each experiment. Also, ferric ion reduction capacity of extracts in presence and absence of chelating agent (EDTA) and total antioxidant capacity were determined. Preliminary phytochemical investigation was carried out to know the nature of constituents present in the leaves and correlate it with antioxidant activity. Further total phenolic content was determined in EJ. $IC_{50}$ values of EJ were 123.09 ${pm}$ 3.01, 643.0 ${pm}$ 61.10, 132.3 ${pm}$ 6.03, 68.5 ${pm}$ 11.5 and 68.13 ${pm}$ 1.38 ${mu}g/mL$ in DPPH radical scavenging, NO scavenging, ${eta}$ CLAMS, OH radical scavenging and anti lipid peroxidation activity respectively. In total antioxidant capacity assay, ascorbic acid equivalent value was found to be 205.56 ${pm}$ 4.69 ${mu}g/mg$ of extract. Total phenolic content was found to be 43.76 ${pm}$ 4.27 ${mu}g$ equivalent of gallic acid per mg of extract. Phytochemical investigation reveals the presence of flavonoids. The results indicate that EJ possess antioxidant activity and flavonoids are responsible for this activity.