Abstract: Based on the fact that methylene blue (MB) was decolorized by oxidants,the color-fading of MB in its reaction with the fenton reagent proved that the hydroxyl free radical produced in the reaction was an oxidant.Furthermore,it was found that the decrease of color intensity of MB was directly proportional to the concentration of hydroxyl free radical and that the linearity between the decrease of color intensity of MB and the amount of fenton reagent added kept quite well when the mole ratio between H2O2 and MB varied in the range 1 to 20.An indirect method of photometric determination of hydroxyl free radical,by measuring the decrease of color intensity (ΔA) of MB at 660 nm,was proposed.This method was also applied to the determination of hydroxyl free radical produced during the process of ultrasonic treatment.It was found that the amounts of hydroxyl free radical produced were directly proportional with the time of ultrasonic treatement.