Abstract: Coke depositions in modern fluid catalytic cracking units (FCCUs) have been found in many parts of the unit, which can form large accumulations in reactors and affect the process performance and more badly, result in a significant safety hazard. Therefore, it is necessary to understand the microstructures of coke and coking process in commercial FCCUs. Coke samples from the FCC reactors in several refineries and their microstructures were analyzed by using both scanning-electron microscope (SEM) and X-ray analysis (EDS) methods. Meanwhile, the coking process was also investigated. The cokes can be divided into four kinds, i. e. filamentous coke, guttate coke, block coke and particle cluster coke, which have different formation mechanisms. The coking process in the FCC reactors is a complicated sequence of catalytic and/or noncatalytic reactions. Some occur in the gas phase and the others on the solid surfaces. The filamentous coke is formed from gaseous hydrocarbons metal-catalyzed reaction in which metal carbides are intermediate compounds catalyzed by iron and nickel. The guttate coke is resulted from the coking of non-volatile tar droplets which are transferred to the solid surfaces. As a result of dehydrogenation and condensation reactions of multi-ring aromatics, resins and asphaltenes and the cyclization and polymerization of olefin and diolefin, the non-volatile droplets are rapidly carbonized and a graphitic coke spheroid is produced. The block coke is formed when the non-volatile droplets are dissolved together and then carbonized. The particle cluster coke is formed by the agglomeration of micro coke particles formed in the gas phase.