FCC catalysts were prepared from Y-zeolite with different unit cell size (UCS). The matrix of the catalysts composed of kaolin filler and silica sol binder, The experimental data clearly show that UCS controls the product distribution of VGO cracking with Y-zeolite catalysts. Coke model was presented to predict coke formation as a function of VGO conversion and catalyst UCS. Gasoline yield was slightly higher at low UCS range (lower than 24.36 Angstrom) while, coke, LPG, and dry gas yields were higher at high UCS range (higher than 24.36 Angstrom). The effect of UCS on hydrogen transfer and isomerization reactions was also investigated.