The study was conducted to determine the effect of sewage sludge components on hydrothermal drying using the sludge of various wastewater treatment plants. According to the tests of equivalent reaction temperature and reaction time the moisture content of sludge can reduce to less than 60%. The solubilization ratio of BJ-Sample, HS-Sample1 and HS-Sample2 at reaction temperature of 190 degrees C were 5.32%, 6.58%, and 10.44%, respectively. There were still about 90% of the materials in the solid phase. It can be said that the characteristics of sludge had no effect on the hydrothermal drying. Thus this showed that hydrothermal drying technology was feasible for the treatment of sewage sludge. The volatile matter and fixed carbon was decreased after hydrothermal treatment, but the ash was increased. It was also observed that with the temperature rising, the volatile matter and fixed carbon were increased gradually and the ash was increased gradually as well. The natural drying characteristics of sludge product were monitored at the same time. From the results we can see that after 24 h the moisture content of sludge was reduced to less than 20%. 0-24 h was the rapid evaporation stage, 24-48 h was slow evaporation stage. The evaporation rate was 4.4 times higher than the untreated sludge samples in the rapid stage. Three different sludge natural drying tests showed that despite the different sludge samples, the results were the same. The optimum reaction temperature and time are 190 degrees C and 30 min respectively. (C) 2011 Published by Elsevier Ltd.