Under microwave irradiation, concentrated fructose (33–92 wt%) in ionic liquids afforded 5-hydroxymethylfurfural in ca. 97–57% yields without addition of catalysts, within 3 min. In-situ13C NMR and 1H NMR spectra suggest that the transformation of fructose in ionic liquid is a highly selective reaction that proceeds predominantly via the cyclic fructofuransyl intermediate. This method is expected to be valuable in facilitating cost-effective conversion of carbohydrates into biofuels and platform chemicals.