Production of bio-oil by microwave-assisted direct liquefaction (MADL) of Ulva prolifera was investigated, and the bio-oil was analyzed by elementary analysis, Fourier transform infrared spectroscopic analysis (FT-IR), and gas chromatography-mass spectrometry (GC-MS). The results indicate that the liquefaction yield is influenced by the microwave power, liquefaction temperature, liquefaction time, catalyst content, solvent-to-feedstock ratio and moisture content. The maximum liquefaction yield of U. prolifera (moisture content of 8%) was 84.81%, which was obtained under microwave power of 600 W for 30 min at 180 degrees C with solvent-to-feedstock ratio of 16:1 and 6% H2SO4. The bio-oil was composed of benzenecarboxylic acid, diethyl phthalate, long-chain fatty acids (C-13 to C-18), fatty acid methyl esters and water. The results suggest that U. prolifera is a viable eco-friendly, green feedstock substitute for biofuels and chemicals production. (C) 2012 Elsevier Ltd. All rights reserved.