The combustion behavior of fixed carbon in supercritical water was explored by a batch reactor. Effects of reaction parameters such as reaction temperature, time, oxygen stoichiometric ratio and stirring rate were investigated. Fixed carbon was difficult to burn out, even if in a harsh condition the conversion of fixed carbon was only 51%. It is determined that for the temperature below 450°C, the process of oxidation was limited by the reaction rate between oxygen and particle surface. For the temperature above 550°C, the rate of oxygen mass transfer to the particle surface was the rate-limiting step. In the transition zone of temperature, surface reaction was comparable to mass transfer in limiting reaction rate. Only trace amount of carbon monoxide was produced at 500°C, and no carbon monoxide was detected at 600°C.