In the present study, the application of hydrodynamic cavitation to improve the biodegradability index of mechanically pre-treated municipal wastewater was examined. The experiments were carried out at the pressure of 3.5, 5, and 7 bar, which was sustained every time in the inlet zone of the cavitation device. As a result, the concentration of soluble chemical and biochemical oxygen demand increased, indicating an effective decomposition and solubilization of complex organic matter. These were accompanied by a visible drop in COD value; in turn, increases in the BOD5/COD ratio known as biodegradability index were observed.