Recently, licorice (Glycyrrhiza uralensis Fisch., GU) has demonstrated significant in vitro and in vivo antioxidant properties. In this study, it was tested that a daily regimen of supplementation with water extract of GU would ameliorate oxidative stress and whether effects were modulated by the glutathion-S-transferase (GST) polymorphisms. Forty healthy male smokers aged 20-60 were divided evenly into 2 groups and given either 260 mL of GU extracted with hot water or 260 mL of oligosaccharide based placebo every day for 8 weeks without any change of usual food intake, and blood samples were drawn before and after the intervention. Eight weeks of GU supplementation significantly decreased plasma conjugated dienes (a maker for lipid peroxidation) in GSTM1 positive subjects, but not in the GSTM1 null genotype group. Our finding suggests that consumption of licorice water extract might be effectively decreasing lipid peroxidation in the subgroup of smokers who have GSTM1 gene.