Controlling beer oxidation using natural antioxidants for better shelf-life stability is the prominent objective of modern breweries. Flavour instability resulting from beer storage and oxidations are the most important quality problems in the brewing industry. This study evaluated the influence of the addition of 80% ethanol leaf extract of Moringa stenopetala to lager beer at 400, 600 and 800 ppm concentrations for 30-, 60- and 90-days storage at room temperature. The total phenolic and total flavonoid contents were determined by Folin-Ciocalteu and aluminium chloride method respectively. Antioxidant activity of each treatment was evaluated and compared by using 1, 1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging activity and total antioxidant using phosphomolybdate assay. The addition of the extract in beer showed a linear increment in the total phenolic content from 46.79 up to 88.30 milligrams of gallic acid equivalent per litre of beer (mg GAE/L) and total flavonoid contents from 123.36 up to 167.09 mg of catechin equivalent per litre of beer (mg CE/L). A similar increment was observed within the DPPH scavenging potential, from 46.55up to 67.16% and total antioxidant power from 139.12 up to 216.67 milligrams of butylated hydroxytoluene equivalent per litre of beer (mg BHTE/L). The result indicates a promising use of leaf extract of M. stenopetala as a functional ingredient in beer to reduce beer oxidation probability and keep its freshness for a period of storage time.