The effects of 6 kinetic parameters such as Y_H (yield coefficient of heterotrophs), b_H (decay rate of heterotrophs, /d), μ_HR (heterotrophic maximum specific growth rate on RBCOD, /d), μ_HS (heterotrophic maximum specific growth rate on SBCOD, /d), K_S (heterotrophic half saturation coefficient for RBCOD, ㎎COD/L) and K_X (heterotrophic half saturation coefficient for SBCOD) on the variation of RBCOD (readily biodegradable COD) concentration were analyzed during the COD fractionation of primary settled municipal wastewater by simulation of respirometric tests. Variation of RBCOD concentration was the highest when Y_H was changed within the acceptable range and standard deviation was about 48% of the averaged RBCOD concentration. Standard deviations of RBCOD concentration were about 10.4 and 6.3% of the averaged values when μ_HS and K_X were changed, respectively and it was about 6.5% when b_H was changed. The effects of μ_HR and K_S on RBCOD estimation were lower than the other kinetic parameters and the variations of RBCOD concentration were only about 2.2 and 3.5% of the averaged value when those two kinetic parameters were changed, respectively. Accordingly, hydrolysis or oxidation of SBCOD (slowly biodegradable COD) was more important than RBCOD oxidation during the analysis of respirometric test. It means that the variation of RBCOD concentration was more sensitive to the change of μ_HS and K_X than that of μ_HR and K_S because μ_HR and K_S did not directly affect the RBCOD concentration in wastewater. However, μ_HS and K_X could affect the RBCOD concentration through hydrolysis of SBCOD.