The FTIR, UV-Visible, 1H NMR and GPC were used to study structural changes of brown-rotted lignin after sodium borohydride reduction. FTIR spectra showed that, after reduction, the band at 1 677 cm-1 attributed to the conjugated carbonyl groups disappeared, and the band intensity at 1 715 cm-1 attributed to the nonconjugated carbonyl groups decreased. On the other hand, the band at 1 509 and 1 603 cm-1 attributed to aromatic skeletal vibration remained almost unchanged. UV spectra showed the decreased absorptions at 288 nm and 300-400 nm after reduction. 1H NMR spectra showed that, after reduction, the number of aromatic methoxyl and aromatic hydroxyls decreased, the number of aliphatic hydroxyls and the proton number attributed to many linkage structures connecting the phenylpropane units increased. GPC results showed that the molecular weight of reduced brown-rotted lignin increased and the molecular distribution got wider, as a consequence of the large molecular weight molecules generated during the reduction reaction. Our results suggest that the conjugated carbonyl groups can be totally reduced to the hydroxyl groups, but only some nonconjugated carbonyl groups can be reduced to the hydroxyl groups. The chemical structure of the brown-rotted lignin changed, but the benzyl ring kept stable. The condensation reaction took place during the sodium borohydride reduction process.