Cultivating microalgae for biodiesel production is a new and emerging research field that has the potential to revolutionize the renewable energy industries. However, up to now, comprehensive study on the conversion efficiency of microalgae lipid to biodiesel is still inadequate. In the present study, reaction optimization studies were carried out to convert high viscosity microalgae lipid (extracted from Chlorella vulgaris) to biodiesel by using sulfuric acid (H2SO4) as catalyst. In order to attain 95% fatty acid methyl ester (FAME) content, the optimized reaction conditions were as follows: methanol to lipid molar ratio of 180, catalyst concentration of 35 wt.%, temperature of 60 degrees C and reaction time of 6 h. Due to the high requirement of methanol and catalyst concentration, attempt was made to improve the reaction conditions by introducing a co-solvent into the reaction mixture. It was found that tetrahydrofuran (THF) could significantly enhance the reaction rate by homogenizing the reaction mixture. The optimum reaction conditions with a co-solvent were found to be less extreme: methanol to THF to lipid molar ratio of 60:15:1, catalyst concentration of 21 wt%, temperature of 60 degrees C and reaction time of 3 h. (C) 2013 Elsevier B.V. All rights reserved.