An efficient microwave-assisted transesterification (MAT) technique was developed to prepare biodiesel from yellow horn (Xanthoceras sorbifolia Burge.) oil with a heteropolyacid (HPA) catalyst namely Cs2.5H0.5PW12O40. A study for optimizing the reaction conditions such as reaction temperature, time, molar ratio of methanol/oil, catalyst amount, and recycle number of catalyst has been performed. The maximum yield of fatty acid methyl esters (FAMEs) reached 96.22% under optimal conditions of temperature 60 degrees C, 10 min, molar ratio of methanol/oil 12:1, 1% (w/w of oil) catalyst and minimum recycle number nine times. The final product of biodiesel, obtained after the new catalyzed process, was analyzed by gas chromatography. The results showed that the Cs2.5H0.5PW12O40 heterogeneous acid catalyst had higher efficiency for transesterification under microwave irradiation compared with the conventional method. The product properties of yellow horn biodiesel are found to be in agreement with EN 14214 standard. (C) 2009 Elsevier Ltd. All rights reserved.