Catalytic cracking of vacuum gas oil (VGO) was carried out in a microactivity test (MAT) unit. Commercial FCC USY catalyst (E-Cat) is used and modified by various zeolites with different pore structures, namely SSZ-74, SSZ-33, MCM-22, MCM-36, ZSM-5, TNU-9, IM-5, and ferrierite to enhance propylene yield. Adding zeolite additives to E-Cat increased catalyst activity. The highest propylene yield of similar to 11 wt.% was achieved over ferrierite and MCM-36 compared with 7 wt.% over E-Cat. An increase in light olefins yield over all additives was associated with a decrease in gasoline yield due to the cracking of gasoline-range reactive species (mainly olefins and iso-paraffins). Zeolite SSZ-74 showed a good enhancement in light olefins yield up to 21.3 wt.% compared with 16.1 wt.% over E-Cat. It is found that additives with lower ability to carry bimolecular reactions are more selective to light olefins. Gasoline quality was improved as the octane number increased over E-Cat with all additives except MCM-36. (C) 2015 Elsevier Ltd. All rights reserved.