A 3-D, three-phase flow-reaction model was developed to predict the performance of FCC riser reactors. This model incorporates the effect of feed vaporization into an earlier 3-D, two-phase flow-reaction model (Gao et al, 1999). The synergistic effects of hydrodynamics, heat transfer, and feed vaporization on FCC reactions are elucidated. The results show that a complex phenomenon occurs in the zone up to 4m above the point at the bottom of the riser reactor where feed is injected. The changes in the spray droplet diameter, volumetric concentration and temperature of the feed in the feed injection zone are illustrated. The model was used as a design tool for an operation modification study. Results indicate that FCC reaction selectivity for the desired products (gasoline and light fuel oil) can be improved by reducing initial droplet diameter in the feed spray from 80 to 40 mum.