Using the energy released in hydrodynamic cavitation to enhance some chemical reaction is a new approach to energy utilization.It is very important to establish quantitatively the relationship between the amount of hydroxyl radical induced in hydrodynamic cavitation and the strength of hydrodynamic cavitation.In this paper methylene blue was used as the catcher for hydroxyl radical induced in hydrodynamic cavitation and an ultraviolet-visible spectrophotometer was used to measure the amount of induced hydroxyl radical.The influences of upstream pipe pressure，liquid temperature and time interval on the amount of induced hydroxyl radical were also experimentally investigated.Meanwhile the suitable temperature and pressure were also established under experimental conditions.The results constituted the fundamentals to quantitatively study the strength of hydrodynamic cavitation and to effectively enhance some chemical reaction.