The municipal wastewater treatment is the source of significant amounts of primary and secondary sludge which is under the present legislation referring to quality and management aspects. It is estimated that a half of wastewater treatment plant costs are due to the sludge management. Anaerobically sludge stabilization, capitalization as energy source, in order to diminish the costs and sludge volume decreasing, are the aims of the main operational steps of sludge treatment, as a part of wastewater treatment plant. The improvement of sludge anaerobically stabilization process must be possible by acting in the rate limiting step - hydrolysis in order to rise the organic carbon solubilization. The increase of soluble carbon can be possible by adding a pretreatment step of waste biological sludge, ultrasonic disintegration being one option. This paper emphasized the experimental results regarding anaerobically stabilization of the thickened waste biological sludge by ultrasonication taking into account the results of blank test, without ultrasonication. Experimental tests show that ultrasonic disintegration of the sludge having initial dried substances content (d.w) 2.72% and soluble organic load COD of 598 mg O2/L led to soluble COD concentration of 4950-6710 mg O2/L after sonication with specific energy in the range of 3.06 - 14.24 kWh/kg d.w. Anaerobically stabilization during 25 test days at 36 0C of the mixture 40% disintegrated biological sludge and 60% digested sludge (inoculum) mixture led to 30-38.6% increase of biogas production comparing with parallel test with non-sonicated sludge.