The effect of steaming temperature on the structural characteristics (e.g. unit cell size, crystallinity and Si/Al ratio), surface area, and acidity as well as on performance of two Y-zeolite based FCC catalysts (coded A and B) having different initial Si/Al ratio was investigated and compared. The feedstock used was vacuum gas oil from Kuwait National Petroleum Co. (KNPC). The results of the study showed that catalyst B with a higher initial Si/Al ratio is better than catalyst A in terms of retaining unit cell size and acidity under hydrothermal conditions. Performance test in a microreactor (MAT) revealed that catalyst B was more active for overall conversion. However, catalyst A was more selective for gasoline yield and RON. It also produced less gas and coke yields than catalyst B.