A novel fluid catalytic cracking (FCC) process, that utilizes a downer reactor, has been developed to enhance the yield of light olefins under high-severity reaction conditions. The effect of heavier feedstock on this high-severity fluid catalytic cracking (HS-FCC) process has been investigated using a small-scale HS-FCC pilot plant (0.1 b/d). Hydrotreated and virgin vacuum gas oils (VGO), hydrotreated and virgin atmospheric residues (AR) were used as test-feeds in a previous study. The yield of desired products, such as gasoline and light olefins, produced from virgin VGO cracking was 79 wt.-%, which is much higher than that obtained from a conventional FCC process. In the case of hydrotreated VGO, the yield of desired products decreased to 76%, however. On the other hand, AR feeds exhibited a performance similar to VGO with a slight increase in coke formation. In this study microactivity test (MAT) results are reported in which the activity and selectivity of the Y-zeolite based catalyst is evaluated. Kinetic modeling was also done based on a four-lump reaction model.