Three types of cation-exchange resins (NKC-9, 001 x 7 and D61) as solid acid catalysts were employed to prepare biodiesel from acidified oils generated from waste frying oils. The results show that the catalytic activity of NKC-9 was higher than that of 001 x 7 and D61. The conversion of the esterification by NKC-9 increased with increasing in the amount of catalyst, reaction temperature and time and methanol/oil molar ratio. The maximal conversion of reaction is approximately 90.0%. Furthermore, NKC-9 resin exhibits good reusability. Gas chromatography-mass spectrometry analysis reveals that the production is simplex and mainly composed of C16:0 (palmitic), C18:2 (linoleic), and C18:1 (oleic) acids of methyl ester, respectively. (C) 2009 Elsevier Ltd. All rights reserved.