Preparation of biodiesel from waste oils catalyzed by a novel Brønsted acidic ionic liquid with an alkane sulfonic acid group was investigated. The acidity and the activity of the ionic liquid were very low at lower temperature when the ionic liquid was crystalloid; they recovered at higher temperature when the crystallized ionic liquid was liquefied. When methanol:oils:catalyst molar ratios were 12:1:0.06, the yield of fatty acid methyl esters can reach 93.5% after the reaction of acidic oil with methanol had taken place at 170 degrees C for 4h. In addition, the ionic liquid had a good reusability and can be easily separated from the biodiesel by simple decantation.