This paper reports optimization of various geometrical parameters of two types of hydrodynamic cavitation reactors(HC) such as venturi type and orifice type. Mostly orifice and venturi nozzles are used as HC reactor, a simple valve can also be used to cause cavitation which depends on geometry and area of opening. Different operating and geometrical parameters such as divergence angle, throat height/diameter to length ratio, number of holes and inlet pressure to the cavitating device were selected to study the inception, growth and dynamics of cavities. In this work, a comprehensive computational fluid dynamics simulation is performed to numerically investigate the 3-D flow behaviors within HC reactors using cavitational model with standard k-ε Turbulence model. The study of different geometries of venturi type HC reactor(like slit, circular and elliptical) shows that 1:1 of the ratio of throat height/diameter to length ando 6.5 of divergence angle is an optimum geometry for best cavitational activity. In case of orifice, 1:3 of the ratio of diameter to length is best for cavitation and an increase in the total flow area increases the cavitational yield. This paper reports optimization of various geometrical parameters of two types of hydrodynamic cavitation reactors (HC) such as venturi type and orifice type. Mostly orifice and venturi nozzles are used as HC reactor, a simple valve can also be used to cause cavitation which depends geometry and area of ​​opening. Different operating and geometrical parameters such as divergence angle, throat height / diameter to length ratio, number of holes and inlet pressure to the cavitating device were selected to study the inception, growth and dynamics of cavities. In this work , a comprehensive computational fluid dynamics simulation is performed to numerically investigate the 3-D flow behaviors within HC reactors using cavitational model with standard k-ε Turbulence model. The study of different geometries of venturi type HC reactor (like slit, circular and elliptical) shows that 1: 1 of the ratio of throat height / diameter to length ando 6.5 of divergence angle is an optimum geometry for best cavi tational activity. In case of orifice, 1: 3 of the ratio of diameter to length is best for cavitation and an increase in the total flow area increases the cavitational yield.