Four different weight grades of sludge granules were dried at constant temperature of 100～200 ℃. Results showed that sludge drying process included four periods: preheating period, constant rate drying period, the first falling rate drying period and the second falling rate drying period. Sludge drying process was a weak intensity drying process at 100～120 ℃, and critical moisture mass fraction was constant. It was a high intensity drying process at 140～200 ℃, and critical moisture mass fraction increased with the drying temperature or granule size increasing. The drying rate was affected with  drying temperature and the quantity of water evaporating heat in a unit surface area. Reduction of granule size can increase specific surface area and drying rate in a unit surface area. There were a quadratic nonlinear relationship between drying time and temperature, and a linear relationship between drying time and sludge surface area．