A 3-D two-phase flow-reaction model was developed for predicting the performance of FCC riser reactors. This model combines two-phase turbulent flow with 13-lump reaction kinetics. The simulated results show that the gas-particulate turbulent reacting flow regime in the riser reactor is extremely complex, especially at the feed inlet zone of the riser reactor: The distributions of velocities, temperature, and yields are not homogeneous in the riser reactor The model predictions were used as a design tool for operational case studies. The optimum yields of desirable FCC products, gasoline and light fuel oil were achieved at a critical riser height that is less than 10 m above the feed inlet. Beyond the critical riser height, excessive cracking reactions resulted in the increased yields of by-products, cracking gas and coke, at the expense of desirable products. Model simulations carried out to determine the parametric effects on product yields suggest that injection of water as a reaction-terminating medium above the critical riser height can be an effective option for optimizing the yields of desirable products.