Hydrothermal carbonisation was used to synthesise hydrothermally ordered mesoporous carbon (HOMC), employing sucrose as the carbon source, a P123 block copolymer as the surfactant and tetraethyl orthosilicate as the silicon source. A high-density sulfonic-acid-based solid acid catalyst (HOMC-SO 3 H) was subsequently prepared via low-temperature sulfonation of the HOMC with concentrated sulfuric acid. Transmission electron microscopy and small-angle X-ray diffraction analyses showed that both the HOMC and HOMC-SO 3 H exhibited an ordered mesoporous structure. The N 2 Brunauer-Emmett-Teller surface areas of the HOMC and HOMC-SO 3 H were determined to be 112 and 87 m 2 /g, respectively, and the average pore sizes were 11.0 and 15.1 nm, respectively. X-ray photoelectron spectroscopy data confirmed that À SO 3 H groups were successfully introduced onto the HOMC surface by the sulfonation process, and a surface acid concentration of 5.9 mmol/g HOMC-SO 3 H was determined by acid-base titration. The HOMC-SO 3 H was applied to the transesterification of waste frying oil with methanol and a fatty acid methyl ester yield of 95 % was obtained after a 5 h reaction at 180°C.