Abstract: Preparation of modified aluminum thrihydroxide at a high concentration by thermal hydrolysis was studied by examining the rheology of ATH suspension. The ATH suspension was a very sticky time-dependent non-Newtonian fluid and its viscosity increased rapidly with the content of ATH. Rheology study was used to select dispersants for the ATH suspension in acid. A polyelectrolyte surfactant was found to decrease the viscosity of the ATH suspension apparently. The maximum content of ATH with added polyelectrolyte surfactant increased to 293 g/L from 78 g/L. Nano modified ATH could thus be prepared by thermal hydrolysis precipitation. The resulting modified ATH had ultra thin rhombic morphology with the beginning of thermal decomposition higher than 350 ℃ and weight loss of 50% approximately.