The over production of free radicals in the human body is known to cause oxidative stress which plays an important role in the pathogenesis of many diseases. Therefore, the search for natural antioxidants would continue to be a major area of research for many decades. This study was to investigate the in vitro antioxidant activities and to determine the phytochemicals present in Cassia sieberiana. The methanol stem bark extract of C. sieberiana was subjected to phytochemical screening and Gas Chromatography-Mass Spectrometry (GC-MS) analysis. Its antioxidant activities were assayed in vitro using nitric oxide scavenging assay, ferric reducing antioxidant potential assay, lipid peroxidation inhibitory assay and 2-diphenyl-1-picrylhydrazyl scavenging assay. The phytochemical screening of the stem bark extract of C. sieberiana showed saponins, alkaloids, flavonoids, steroids, tannins, phlobatannins, terpernoids, cardiac glycosides, phenolics and reducing sugar. The GC-MS Table revealed that the methanol extract has 51 bioactive compounds with 3-0-Methyl-d-glucose having the highest percentage of 12.16 % while 5.alpha-Androstan-3-beta-ol,4,4-dimethyl- having the lowest percentage of 0.09 %. The antioxidant study showed concentration-dependent radical scavenging activity of the methanol stem bark extract of C. sieberiana. This concentration-dependent result was also observed when compared with ascorbic acid used as the standard drug. The presence of different phytochemicals such as alkaloids, saponins, tannins, phenols, flavonoids, among other things, might be responsible for C. sieberiana antioxidant activity. The study concluded that the methanol stem bark extract of C. sieberiana contains 51 bioactive compounds and showed good antioxidant activity which could be therapeutic in several human diseases.

Key words: Cassia sieberiana, Phytochemical, Antioxidant, Pharmaceuticals