Abstract: A novel basic ionic liquid paired by 1-butyl-3-methyl-morpholine cation and morpholine anion (Im) was synthesized in two steps; the structure of ionic liquid intermediates was characterized by ¹H-NMR and FT-IR. The alkaline ionic liquid was obtained through anion exchange and used as catalyst in the synthesis of biodiesel by transesterification. The results showed that the alkaline ionic liquid Im is highly active as a catalyst in the transesterification; under the optimum reaction conditions of 60 ℃, Im amount of 3%, and methanol/oil mol ratio of 6.5：1.0, the content of fatty acid methyl ester (FAME) in the product reaches 95.80% after reaction for 2 h. Moreover, the alkaline ionic liquid exhibits high catalytic stability upon reusage; it remains highly active even after being reused for five times.