Thermal pretreatment can be applied to sludge anaerobic digestion or dewatering. To analyze the variation in humic substances during thermal sludge treatment, sludge humic and fulvic acids were extracted before and after 30-min thermal treatment at 180°C, and then their contents, molecular weight distributions, elementary compositions, and spectral characteristics were compared. The results showed that the total contents of humic and fulvic acids in the sludge almost remained constant during thermal treatment, but 35% of humic and fulvic acids were dissolved from the sludge solids. Moreover, both humic and fulvic acids were partly decomposed and 32% of humic acids were converted to fulvic acids. The median value of the molecular weights of humic acids decreased from 81 to 41 kDa and that of fulvic acids decreased from 15 to 2 kDa. Besides the reduction in molecular size, the chemical structures of humic and fulvic acids also exhibited a slight change, i.e. some oxygen functional groups disappeared and aromatic structures increased after thermal sludge treatment.