Microwave-assisted direct liquefaction (MADL) of Ulva prolifera was performed in ethylene glycol (EG) using sulfuric acid (H2SO4) as a catalyst. Response Surface Methodology (RSM) based on central composite rotatable design (CCRD) was employed to optimize the conditions of three independent variables (catalyst content, solvent-to-feedstock ratio and temperature) for the liquefaction yield. And the bio-oil was analyzed by elementary analysis, Fourier transform infrared spectroscopic analysis (FT-IR) and gas chromatography-mass spectrometry (GC-MS). The maximum liquefaction yield was 93.17%, which was obtained under a microwave power of 600 W for 30 min at 165 degrees C with a solvent-to-feedstock ratio of 18.87:1 and 4.93% sulfuric acid. The bio-oil was mainly composed of phthalic acid esters, alkenes and a fatty acid methyl ester with a long chain from C-16 to C-20. (C) 2013 Elsevier Ltd. All rights reserved.