Hydrothermal treatment was carried out with sewage sludge, and the effect of hydrothermal condition, such as treatment temperature, treatment periods and slurry concentration, on sludge slurry characteristics was evaluated. In the experiment, treatment temperature was changed 393 to 533 K, treatment period was changed 5 to 20 min and slurry concentration was changed around 14.3 to 33.3%. Elementary analysis, particle size distribution measurement and zeta potential measurement of hydrothemally treated slurry were conducted. pH value of the filtrate was also measured. Constant pressure filtration experiment with the slurry was implemented to evaluate dewatering characteristics of the treated slurry. With increasing treatment temperature, solubiliztion ratio of the sludge increased significantly, and 60% of organic material in sewage sludge solubilized into water at the treatment temperature of 533 K. Effect of treatment period on solubilization ratio was minor comparing with the treatment temperature. Due to the solubilization of sludge components, particle diameter decreased with increasing treatment temperature. pH value of the filtrate also decreased with increasing temperature, however, the value increased significantly over 473 K treatments. The property of treated slurry and ph value of the filtrate changed significantly by the hydrolysis and dehydration reaction during the hydrothermal treatment, absolute value of zeta potential also changed obviously. The absolute zeta potential value had a minimum value at 473 K, which was observed similarly in pH value trend. Dewatering ability of slurry treated over 473 K improved drastically and dewatering rate increased with increasing the treatment temperature. Solid content of dewatering slurry became around 52%, which capable of combustion without drying.