Abstract: Modification of the standard method for determination of COD of waste water (GB/T 11914-1989) was made by using Mn2+ ion instead of Ag+ as catalyst and using a small amount of Ag+ ion instead of Hg2+ as masking agent for Cl- ion.By this modification,the time of oxidation was shortened from 2 h to 6 min,and thorough elimination of the use of mercuric ion as masking agent was attained.The optimum conditions of the modified method were as follows:temperature of heating:160 ℃;time of refluxing:6 min;acidity:15 mol·L-1;amount of manganese sulfate added:7.5 g·L-1 and lower limit of determination:100 mg·L-1 COD.