The compound 5-hydroxymethylfurfural (5-HMF) is thought to be an intermediate for the formation of tarry material in the supercritical water gasification (SCWG) of biomass. To gain insight on the formation mechanism of tarry material, we examined 5-HMF under both subcritical and supercritical conditions. The experiments of this study were conducted in a continuous reactor designed for an instant heating of 5-HMF solution to the reaction temperature. The reaction temperatures were set at 350 and 450 degrees C, and the pressure was set at 25 MPa. The initial concentration of 5-HMF and the residence time were adjusted in the range of 0.02-0.15 M (at room temperature) and 80-3000 s, respectively. The formation of char particles was only observed under the subcritical conditions. Char yield increased as the initial concentration of 5-HMF rose, with a reaction order of 4.29, with respect to 5-HMF concentration. The liquid products obtained from the 5-HMF decomposition also contributed to the polymerization to form char, with a reaction order of 1.21, based on their concentration. The sensitivity analysis indicated the high sensitivity of reaction orders on the char yield. The liquid products were identified by liquid chromatography/mass spectroscopy (LC/MS) and high-performance liquid chromatography (HPLC). The char particles were characterized by scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) spectroscopy, and Raman spectrometry. The char characterization results implied that the char particles formed had a polyaromatic structure. Based on our experimental findings, we proposed a mechanism of char particle formation from 5-HMF.