Two acidic ionic liquids based on 1-benzyl-1H-imidazole have been synthesized and characterized by FT-IR, H-1 NMR, C-13 NMR and CHNS analyses. The ionic liquids have been used for transesterification of Nigella sativa seed oil as newer sources of oil with methanol. We determined relative fatty acid content, iodine and saponification values of seed oil and estimated cetane number of produced biodiesel based on the chemical composition of oil. Also, the effect of reaction temperature, type and amount of catalyst, molar ratio and reaction time were investigated. The results show that the ILH2 ionic liquid has the higher catalytic activity under the optimized reaction conditions. Thus, this ionic liquid is able to catalyze the transesterification of N. sativa seed oil to its methyl esters in 6 h with yields of more than 93%. The catalytic activity of ILH2 for the transesterification of other vegetable oils and alcohols has also been studied. (C) 2013 Elsevier B.V. All rights reserved.