An expression of adult wing form in reaction to rearing density during nymphal stage was investigated in the brown planthopper, Nilaparvata lugens. Under mass rearing condition, the S-BPH and the 2007-BPH population predominantly showed a short-winged, brachypterous, form and a long-winged, macropterous, form, respectively. At rearing density of less than 5 nymphs in the 2007-BPH, 90% of females showed brachypterous form, but all males became macropterous form. The ratio of macropterous form in the 2007-BPH males decreased by 75% in 10 to 15 rearing density, but increased by more than 95% again at 20 to 30 rearing density. In the case of the 2007-BPH females, the ratio of macropterous form gradually grew from 31% at 10 nymphal density to 92% at 20 nymphal density. All females originated from the S-BPH showed brachypterous form, regardless of nymphal density. The ratio of macropterous males in the S-BPH rapidly went down from 74% at 1 nymphal density to 10% at 10 nymphal density. At 20 nymphal rearing density, all males of the S-BPH showed brachypterous form. On the other hand, other brachypterous (OJ67-BPH) and macropterous (2006-BPH) population produced similar results with above the two populations at 1 nymphal rearing density. In summary, these results demonstrate that wing form dimorphism in N. lugens is largely influenced by nymphal density and the wing form at a specific density (low or high) can be different by sex or N. lugens populations.