Feedstock atomization affects catalytic cracking reactions that take place in an FCC riser reactor, and is considered a critical parameter especially when processing heavy fuels. A finely atomized feed allows instantaneous flash vaporization and quick quenching of the catalyst, conditions that allow the feed components to be subjected to an ideal mixing temperature. Mixing conditions at the inlet of the riser reactor also affect cracking reactions selectivity. An atomization modeling scheme has been incorporated into a CFD model developed by Theologos & Markatos (1993, A.I.Ch.E. Journal, 39(6), 1001.) and Theologos, Nikou, Lygeros & Markatos (1997, A.I.Ch.E. Journal, 43(2), 486) and has been used to evaluate atomization effects on feedstock vaporization rates, cracking reactions initiation, reactor selectivity and overall reactor performance. Cracking reactions are simulated using a simplified 3-lump model. The atomization modeling scheme takes into account the initial feed droplet size at nozzle's exit, as well as droplet size reduction along with vaporization. Droplet vaporization parameters from the literature have been considered. To evaluate the degree of feedstock atomization effects on reactor performance, a parametric study is carried out considering three different initial feed droplet sizes, namely 30, 100, and 500 mu m. The area where feedstock vaporization takes place inside the reactor is predicted and the evolution of atomized droplet size is illustrated. The effect of the initial degree of atomization on cracking reactions selectivity is discussed.