The objective of this research was to examine the influence of energy input on the particle size distribution of disintegrated sludge. The change of particle size distribution indicates the deagglomeration of flocs and disruption of micro-colonies. As the digestibility of sludge increases with dispersion, particle size analysis is an important factor in evaluating the disintegration process. Four different levels of energy input were used in the research: 10–100 kWh·m−3. All samples showed significant changes as far as dispersion (kdCST = 22.98–74.67, kdFCOD = 3.23–18.46), lysis (kdSCOD = 4.22–12.09), acidification (kdVFAs = 1.78–12.61), nitrogen release (kdTN = 4.02–21.61) indicators were concerned. Results indicate the gradual decrease of measured particle size with increasing energy input. The energy supplied to the disintegration process primarily promotes deagglomeration and with the rise of energy input, the destruction of cells. For EV = 50 and 100 kWh·m−3 an increased occurrence of lysis effects and increase in particle fraction <99.9 μm was noted. The highest efficiency evaluated by increase of filtered chemical oxygen demand (FCOD) and soluble COD (SCOD) per unit of volumetric energy – ΔCOD and ΔSCOD (mgO2·Wh−1) was obtained for Ev = 10 WhL−1, which corresponds to the most significant change in particle size distribution. The volume of particles <99.9 μm rose from 1.92% for non-disintegrated sludge to 26.62% for volumetric energy 100 kWh·m−3.