Determination of chemical oxygen demand using permanganate (COD_Mn) is frequently used as an index of water pollution caused by the effluents from factories. But this oxidation reaction is incomplete and special heating condition for the official methods should be maintained. Then, in this report, a semimicro method of COD determination using 20 ml of sample water was devised and heating condition was checked with time course of elevating temperature of test solutions. COD values of dicarboxylic acids, or amino acids increased gradually with heating time and detection of small difference of heating condition could be detected. In these methods, good agreement of COD values of semimicro method to the ones of the official method could be obtained, so the improved semimicro method is applicable to the COD_Mn determination of various sample waters very efficiently.