Absolute methanol extract leaf of Costus afer was comprehensively investigated for free radical scavenging activities and antioxidant activities using battery of testing: 1, 1-diphenyl-2-picrylhydrazyl (DPPH) scavenging, hydroxyl radical scavenging, hydrogen peroxide radical scavenging, 2, 2′-azino-bis (3-ethylbenzothiazoline-6-sulphonic acid) (ABTS) scavenging, nitric oxide inhibition, anti-lipid peroxidation, ferrous chelating potential, reducing power potential, total antioxidant capacity, cupric reducing antioxidant capacity CUPRAC and ferric reducing antioxidant power (FRAP). The content of phenol, flavonoid, cupric, anthocyanin were also investigated. The results suggest that the antioxidant potentials of the extract may be responsible for its medicinal potentials. Cell viability assay revealed that up to12 μg/ml of the extract is safe for Chang liver cell and no sign of toxicity was observed after the extract treatment of 2000 mg/kg in albino rats.