Response surface methodology (RSM) was used to optimize the microwave-assisted direct liquefaction of Sargassum polycystum C.Agardh in ethylene glycol (EG) with H2SO4 as a catalyst. Based on the results of single factor experiments, EG-to-feedstock ratio, temperature and catalyst content were chosen as independent variables for a central composite rotatable design (CCRD). The optimal liquefaction conditions were estimated as: the EG-to-feedstock ratio of 18.50:1 (w/w), the temperature of 170 degrees C, the reaction time of 15 min, catalyst content of 9.6% (catalyst/EG, w/w%) and microwave power of 400 W with the liquefaction yield of 87.70%. The bio-oils were mainly composed of fatty acid methyl ester and alkane with a long chain from C-17 to C-20. (c) 2012 Elsevier Ltd. All rights reserved.