In general, growing particle deposit in reactor cyclone duct has caused operational safety problems as well as a decrease of the cyclone efficiency. In this paper, Computational Particle Fluid Dynamics (CPFD) simulation for forecasting the behaviors of particle deposit formation in cyclone duct in Residue Fluidized Catalytic Cracking (RFCC) process is carried out. The CPFD simulation based on Multi-Phase Particle In Cell (MP-PIC) method in which particle phase is based on a stochastic particle model using the Lagrangian method and fluid phase based on an Eulerian method. We aim at analyzing the flow patterns of both particles and fluids, depending on the different sizes and shapes of the cyclone duct. As a result, we found that low speed zone existed on the duct 90° (inlet position 0) at the initial time. Some particles passing out of main flow path tend to stick to the wall of the cyclone duct due to its low speed. A set of regions where particle deposit formation takes place are set as a start point for CPFD simulation. The deposit grew up to the size of 90mm thick, and sometimes fell off due to scouring phenomenon, which results in plugging in the dipleg, leading to serious problems such as carryover.