Catalytic oxidation of lignocellulose in hydrothermal condition catalyzed by vanadium-containing homogeneous catalysts is a promising method to prepare formic acid (FA), a potential material in emerging field of clean energy technologies. The FA production from polysaccharides components of lignocellulose has already been well studied. However, the conversion of lignin component, which quite differs in structure from polysaccharides, has not been studied systematically. In this work, the lignin degradation was studied in NaVO3-H2SO4 aqueous solution with O-2 as oxidant. The degradation mechanism was studied in detail. Compared with polysaccharides, lignin can produce a much lower yield of FA with a formation of a larger amount of CO2. H2SO4 can catalyze highly-polymerized lignin to segments by C-O bond hydrolytic cleavage, but the conversion is limited. NaVO3 and O-2 can accelerate the lignin degradation by weakening the C-O bond. In liquid phase, NaVO3 and O-2, with a low selectivity on products, lead to different kinds of reactions, mainly including phenolic hydroxyl oxidation to quinone, aliphatic hydroxyl oxidation to ketone/aldehyde, aliphatic C-C. bond cleavage, intramolecular dehydration and decarboxylation. This study gives a deep understanding of lignin transformation during catalytic oxidation, and provides guidance to the further application of FA production from natural lignocellulose. (C) 2017 Elsevier B.V. All rights reserved.