AbstractA novel reaction route was proposed from 5-hydroxymethylfuran-2- carbaldehyde (HMFCA) to 2-hydroxy-5-methylene-2,5-dihydro-furan-2-carbaldehyde (HMDFC) on the basis of the mechanism previously offered by Horvat, to account for the formation mechanism of levulinic acid. The probabilities of the two mechanisms were compared by Gaussian 03 software. It was found that the conversion from HMFCA to HMDFC in the newly deduced mechanism has a lower net energy requirement than that in the original mechanism, and thus should be more preferable. The mechanism indicates that HMFCA is initially protonized by H+ addition at the position 5 of the furan ring, and then combines with OH-, thereby completing the hydration process after isomerization. Finally, an H2O molecule is released, forming the intended intermediate product of HMDFC.