1-Butyl-3-methylimidazolium hydrogen sulfate ([Bmim][HSO4]) is used as a solvent and an acid catalyst for in-situ extractive transesterification of wet Nannochloropsis with methanol. The reaction is supposed to be a five-step process: (1) wet algae cell wall dissolves in ionic liquid at reaction temperatures; (2) hydrogen ions and sulfate ions release from the ionization of HSO4. The hydrogen ions (H+) act as catalysts for accelerating the reactive extraction of triglyceride from wet Nannochloropsis; (3) hydrogen ions and methanol molecules transfer from bulk to active site of cells without passing through cell wall that is dissolved by ionic liquid; (4) in-situ transesterification of lipid (mainly triglycerides) with methanol; and (5) products transfer from inside of algae cells to outside of cells. The crude biodiesel yield of [Bmim] [HSO4] catalyzed in-situ transesterification is about 95.28% at reaction temperature of 200 degrees C, reaction time of 30 min, mass ratio of [Bmim][HSO4] to wet Nannochloropsis of 0.9:1, and a mass ratio of methanol to wet algae of 3:1. It decreases to 81.23% after [Bmim][HSO4] is recycled for 4 times, which indicates that [Bmim][HSO4] catalyzed in-situ transesterification is an economic approach for biodiesel production from wet algae. (C) 2016 Elsevier Ltd. All rights reserved.