Rheological characterization of sludge is essential, since it is related to access sludge transportation and mixing performance, which is included in many processes in waste water treatment and sludge disposal. This study was carried out for characterizing the secondary sewage sludge after gravitational thickening from a Dutch wastewater treatment plant (WWTP), which was also concentrated by vacuum filtration. The measurements of flow curves and yield stress were carried out under conditions of different solid concentrations (TS) and temperatures. Both the Ostwald and Herschel-Bulkley (HB) models were used for fitting the measured data; and a hybrid model was recommended for describing the flow behavior in a wide range of shear rates. HB model was better for fitting the flow curve of the raw sludge within the whole range of shear rates and only at lower shear rates of the concentrated sludge. Moreover, Ostwald was better for the flow curves at higher shear rates of the concentrated sludge. Regarding the parameters in the model fitting, K and n values were in certain relationship with the temperature of each sample. A power law or exponential relationship was used for describing the correlation between the yield stress and the TS.