The purpose of this study was to investigate the free-radical scavenging activity and identify the chemical composition of Clematis flammula leaves. The phenolic composition of Clematis flammula leaves from the Algerian region was investigated. Extraction was performed in ethanolic solvents. Their efficiency was determined in terms of the quantitative composition in phenolic compounds by HPLC-DAD of the extract isolated from Clematis flammula leaves. Its leaf extract has been found to contain a high concentration of phenolic compounds, which include trans chalcone (1.82 ± 0.18 µg/ml), 2,3-dihydroflavone (0.97 ± 0.26 µg/ml), 3-hydroxy flavone (0.85 ± 0.15 µg/ml), cinnamic acid (0.81 ± 0.11 µg/ml), catechin hydrate (0.52 ± 0.04 µg/ml), and trans hydroxy cinnamic acid (0.38 ± 0.09 µg/ml), which are the main constituents. There are also six existing minor acidic compounds and flavonoids, such as rutin hydrate, chlorogenic acid, and ellagic acid. The highest yield of phenolic compounds from Clematis flammula was found in the ethanolic extract. On the other hand, the ethanolic extract exhibited a high scavenging activity against DPPH inhibitory activity (IC50 = 7.56 µg/ml) when compared with standard BHT (IC50 = 0.20 µg/ml).