Allamanda cathartica is a herbal plant in the Apocynaceae family, often known as "Wel Rukaththana" in Sinhala, "Thimble Lady," or "Golden Trumpet" in English. Traditional medicine has utilized this plant's leaves, roots, stems, blossoms, and entire plant to cure a variety of illnesses since ancient times. Even though A. cathartica is a commonly used herb, the literature review indicated that there is a scarcity of published scientific evidence about its therapeutic usefulness. Hence, the present study aimed to evaluate the comparison of antioxidant activity of fresh leaves, roots, stems and flowers of this pant by using 1, 1-diphenyl-2-picrylhydrazyl (DPPH) assay. Fresh plant materials were collected from the Colombo district and authenticated. Hot aqueous extracts of fresh leaves (AEFL), roots (AEFR), stems (AEFS) and flowers (AEFF) were prepared with 3 g/mL concentrations. In-vitro antioxidant activity of the concentration gradients of each extract was evaluated by using a DPPH assay. Ascorbic acid (6.25 μg/mL- 25 μg/mL) was used as a positive control. The radical scavenging activity of test samples was expressed as an EC50. The hot aqueous extract showed antioxidant activity with an EC50 value of 10.92 μg/mL, 22.10μg/mL, 23.76 μg/mL and 27.38 μg/mL for leaves, flowers, roots and stems respectively while the ascorbic acid showed an EC50 value of 13.40 μg/mL. In conclusion, the results showed that AFEL has a significant (p < 0.05) antioxidant potential than AEFR, AEFS and AEFF. Hence, fresh leaves of A. cathartica have been identified as the most potential part for antioxidant activity among tested plant parts.