Abstract: The catalyst system of cobalt, manganese and bromine was used in the process of p-xylene (PX) liquid phase oxidation. The catalysts exhibited a synergistic effect during reaction. To investigate the distinctive role of each catalyst component, a series of PX oxidation experiments in different catalyst conditions such as Co/Mn ratios, bromine and total catalyst concentrations were carried out. The fractional kinetic model was used to fit the data and the rate constant for each oxidation step was obtained. The result showed that, in the experimental conditions under consideration, there existed an optimum ratio of cobalt vs manganese (Co/Mn=1) and the process was more sensitive to the change of bromine concentration than that of cobalt or manganese.