The synthetic effect of the combination use of ultrasound irradiation (UI) and ionic liquid (IL) on improving enzyme activity was studied when they were employed in isomerization of glucose to fructose by immobilized glucose isomerase (IGI, produced from streptomyces murinus and immobilized on silica). Both of UI and IL [EMIM][CI], which was screened as the best medium for this reaction, were found to increase the enzyme activity in isomerization reaction. And a further increase of enzyme activity was observed by combination use of UI and IL. A systematic screening and optimization of the reaction parameters in ILs under UI on the IGI activity were performed. Under the optimum reaction conditions. 45.3% yield of fructose was achieved in 10 h under UI in [EMIM][CI], compared to only 41.5% yield under stirring in [EMIM][CI], 44.2% under UI without [EMIM][CI] and 38.9% under stirring without [EMIM][CI] in 12h. respectively. High thermal stability and reusability of IGI was also observed under UI [EMIM][CI]. These results indicated that the combination use of UI and IL might be a fast and efficient method for enzymatic isomerization of glucose to fructose. (C) 2012 Elsevier Ltd. All rights reserved.