Nano-MgO can apparently improve the transesterification reaction of soybean oil with supercritical/subcritical methanol. The variables affecting the yield of methyl ester during the transesterification reaction, such as the catalyst content, reaction temperature and the molar ratio of methanol to soybean oil were investigated and compared with those of non-catalyst. When nano-MgO was added from 0.5 wt% to 3 wt%, the transesterification rate increased evidently, while the catalyst content was further enhanced to 5 wt%, little increased in yield. It was observed that increasing the reaction temperature had a favorable influence on methyl ester yield. In addition, for molar ratios of methanol to soybean oil ranging from 6 to 36, the higher molar ratios of methanol to oil was charged, the faster transesterification rate was obtained. When the temperature was increased to 533 K, the transesterification reaction was essentially completed within 10 min with 3 wt% nano-MgO and the methanol/oil molar rate 36:1. Such high reaction rate with nano-MgO was mainly owing to the lower activation energy (75.94 kJ/mol) and the higher stirring. (c) 2006 Elsevier Ltd. All rights reserved.