Deposits in a commercial RFCC reactor were characterized using an analytical approach. Further characterization of deposits on the reactor cyclone, which are one of the primary problems causing abnormal shut-down of RFCC, was carried out in order to understand the formation mechanism of the deposit. The deposits in the reactor consist of hydrocarbon and inorganic matter, compositions of which are well matched with the RFCC catalyst. In a longitudinal analysis of a sample reflecting age, H/C atomic ratio decreases slightly with increased aging time in inner regions and has high value at the surface region, which is a relatively fresh deposit. The particles, surrounded by carbonaceous matter, and amorphous carbonaceous matter are observed through surface micrographs. Aromatic compounds and nano- and microcatalyst fines contribute to the formation of deposits. Micro- and nanostructure of the deposit is amorphous or very weakly ordered. The H/C atomic ratios of samples with different ages, over 90 days, do not change with aging and are matched with a variation of feed API. Possible mechanisms for the deposit formation and its reducing methods are proposed.