Corncob lignin was extracted with formic acid /acetic acid aqueous solution. The Response Surface Methodology (RSM) analysis was used to study the effects of process parameters on the lignin yield and the optimized condition (formic acid/acetic acid ratio of 4/5(V/V), 91 ℃ and 4 h) was established. The highest yield of lignin was predicted to be 67.91 % by the RSM model and revised to be 70.16 % by the experiments. The purity of obtained lignin, the contents of phenolic hydroxyl and carboxyl group were determined using differential UV-Spectroscopy method ( method) and chemical titration. The complex structures of the obtained lignin were further characterized by FTIR, UV, TG, 1HNMR and compared with that extracted with alkali. The results showed that the lignin is composed of guaiacyl unit (G), syringyl unit (S) and p-hydroxyphenyl unitl (H) and is quite similar with the alkali one in structure. The thermogravimetric analysis showed that the lignin extracted with organic acid queous solution is thermally more stabile than the alkali one and the char residues at 600 ℃ was 42 %. Extraction of lignin from corncob with organic acid aqueous solution was proved to be an efficient method.