This study was carried out to determine the polyphenolic contents and antioxidant activity of hydroalcoholic extract of Gutenbergia nigritana leaves using in vitro models. The ability of the extract to scavenge free radicals such as 2,2-diphenyl-1-picrylhydrazyl (DPPH•), hydrogen peroxide (H2O2) and nitric oxide (NO) was investigated. The ferric reducing antioxidant power, total phenols and flavonoids contents of the extract were also determined using spectrophotometric methods. The result showed that the extract scavenge the radicals in a concentration dependent manner. The Free radical scavenging potentials of the extract was found to be proportional to its polyphenolic contents. Our findings revealed that the leaves of Gutenbergia nigritana contain biologically active constituents and therefore may be used as a potent antioxidant for the management of radical related diseases.