Supercritical water oxidation (SCWO) progress for a spherical activated carbon particle was pursued using a flow-type reaction cell equipped with sapphire windows, where the change in the size and shape of the particle inside the cell was observed in real time by means of a shadowgraph. While maintaining its shape, the particle radius decreased almost linearly with time at a rate on the order of micrometers per second. Phenomenologically, the reaction occurred at the surface of the sphere. The decreasing rate of the radius was nearly first order with respect to the O-2 concentration and was almost independent of the temperature and pressure in the cell. On the other hand, the rate increased with the flow rate of the supercritical water. These trends suggested that the reaction was almost limited by the mass transfer of O-2 from the bulk to the surface of the particle. The observed rate roughly agreed with the rate of O-2 mass transfer which was estimated using empirical relations.