The microstructure and properties of the coke samples collected from 4 different wall regions of the cyclone in the reactor of a residue lfuid catalytic cracking unit (RFCCU) were analyzed by using the scanning-electron microscope (SEM), and the possible coke formation processes were investigated as well. The results showed that some of the heavy nonvolatile oil droplets entrained in the lfowing oil and gas mixture could possibly deposit or collide on the walls by gravity settling or turbulence diffusion, and then were gradually carbonized into solid coke by condensing and polymerization along with dehydrogenation. Meanwhile some of ifne catalyst particles also built up and integrated into the solid coke. The coke can be classiifed into two types, namely, the hard coke and the soft coke, according to its property, composition and micro-structure. The soft coke is formed in the oil and gas mixture’s stagnant region where the oil droplets and catalyst particles are freely settled on the wall. The soft coke appears to be loose and contains lots of large catalyst particles. However, the hard coke is formed in the oil and gas mixture’s lfowing region where the oil droplets and catalyst particles diffuse towards the wall. This kind of coke is nonporous and very hard, which contains a few ifne catalyst particles. Therefore, it is clear that the oil and gas mixture not only carries the oil droplets and catalyst particles, but also has the effects on their deposition on the wall, which can inlfuence the composition and characteristics of deposited coke.