The present work studied the antioxidant capacity of the lignin obtained from Miscanthus sinensis. As the lignin structure is very complex, extraction and separation processes highly influence obtained lignin structure and its properties, including the antioxidant activity. Lignin fractions were obtained from black liquor resulting from different fractionating and autohydrolysis processes of M. sinensis. Different lignin fractions with specific molecular weight were separated by ultrafiltration using different cut-off ceramic membranes (10 and 15 kDa). The physico-chemical properties, phenolic groups content and the antioxidant capacity of each lignin sample were studied. It was found that antiradical activity of the analyzed lignin samples was closely related with the used fractionating process (soda, organosolv and autohydrolysis treatments). The ultrafiltration process allows obtaining lignins with different antioxidant capacity, improving this property as the used cut-off was greater. (C) 2010 Elsevier Ltd. All rights reserved.