This paper presents the application of a monitor technique using electrical resistance tomography (ERT) for controlling the optimum polymer dosage for sludge dewatering. The content starts from introduction for characterization of a raw sewage sludge sample, including particle size distribution and volumetric solids concentration, and the conductivity properties of the sludge and the polymer used. The experimental results demonstrate that the different polymer doses mixed with raw sludge leads to a change of floc size and distribution, and further these changes can be monitored using ERT. The result of ERT measurement can be applied in determining the optimum polymer dosage in the water treatment, as a potential new technique.