Abstract: The antioxidant activity of the methanolic and aqueous extracts of Martynia annua Linn. leaves were evaluated by several in vitro systems of assay, namely, reducing power assay, DPPH radical-scavenging activity, nitric oxide scavenging activity, H2O2 radical scavenging activity, superoxide radical scavenging assay, hydroxyl radical-scavenging activity, and total antioxidant capacity. Total phenolic content was measured by Folin–Ciocalteau reagent. The antioxidant property depends upon concentration and increased with increasing amount of the extract. The free radical scavenging and antioxidant activities may be attributed to the presence of phenolic and flavonoid compounds present in the extract. The results showed that the methanolic extract exhibited higher antioxidant activity than the aqueous extract. Chlorogenic acid is reported in this plant and a TLC densitometric method was developed for the quantification of Chlorogenic acid.