The effect of the unit cell size of Y zeolites on their cracking performance has been investigated by X-ray diffraction, nitrogen adsorption, and VGO cracking reaction using a micro-activity test unit. Y zeolites with unit cell size in the range 24.21-24.60 Angstrom were used as materials for fluid catalytic cracking. The matrix of the catalysts composed of kaoline filler and silica binder is quite inactive for VGO cracking. The intrinsic activity of the catalysts, defined as the rate constant of VGO cracking divided by the zeolite surface area, increases with increasing unit cell size and then decreases through a maximum and correlates directly with the number of 0-NNN aluminum atoms. This indicates that only 0-NNN aluminum atoms in the zeolite are the active sites responsible for VGO cracking. The increased coke yield with larger unit cell sizes can be attributed to the increased number of 1,2,3,4-NNN aluminum atoms.