The effect of two-stage thermal disintegration of sewage sludge on the particle size distribution using laser diffraction method has been studied. The sludge was sampled from municipal sewage treatment plant after each stage of disintegration. The first stage of disintegration known as homogenization proceeds at temperature of 70-90°C and pressure of 3 bar, the second stage called thermal hydrolysis was performed at temperature of 160-170°C and pressure of 6 bar. It was found that the first stage of disintegration has the strongest impact on the reduction of the sludge particle size and changes in chemical properties. The maximum size of the particles from raw sewage before disintegration was 310 μm. After first stage of the process average size of the particles was 250 μm, and during the second stage it was reduced to 226 μm. Sludge disintegration degree (DDCOD) of 59% confirms high effectiveness of the process. We established that the redox potential (Eh) of sludge effluents was changed after each step of the studied process. Furthermore, chemical oxygen demand (COD) increases which leads to the conclusion that resizing of floccules is accompanied by hydrolysis.