To improve beer flavour stability by adding chitooligosaccharides that prevent formation of staling compounds and also scavenge radicals in stale beer. Chitooligosaccharides, at 0.001-0.01%, inhibited the formation of staling compounds in forced aged beer. The formation of 5-hydroxymethylfurfural, trans-2-nonenal and phenylacetaldehyde were decreased by 105, 360 and 27%, respectively, when compared with those in stale beer without chitooligosaccharide addition. The capability of chitooligosaccharides to prevent staling compound formation depended on their molecular size (2 or 3 kDa). The DPPH/hydroxyl radical scavenging activity in fresh beer significantly lower than that in forced aged beer in the presence of chitooligosaccharides. When compared with stale beer without added chitooligosaccharides, the radical scavenging activity could be increased by adding chitooligosaccharides to forced aged beer. Chitooligosaccharides play an active part in the prevention of beer flavour deterioration by inhibiting the formation of staling compounds and increasing radical scavenging activity.