The measurements of rheological properties of activated sludge suspension after mechanical disintegration indicate a substantial decrease in viscosity of up to 60%. Together with the decrease of viscosity pseudoplastic properties increased and the flow limit dropped. Similarly, significant changes of rheological properties occurred in the sludge, subjected to methane fermentation. One can observe that the length of fermentation period influences the decrease in viscosity of the sewage sludge. It is postulated to use the measurement of viscosity for the estimation of excessive sludge disintegration ratio as this method is much faster than determination of COD.