This study considers the effects of diffusion and intrinsic catalytic reactivity for the cracking of gas oil on FCC catalysts. A four-lump kinetic model, initially developed in a novel CREC Simulator, is employed to estimate the unconverted gas oil, the gasoline, the light gases and the coke fractions. The model incorporates a modified Thiele modulus, based on zeolite crystal properties, and an effectiveness factor. The calculated effectiveness factor is found to increase with the gas oil conversion and to decrease with the intrinsic rate constant and the zeolite crystal size. The proposed model predicts that crystal size changes in the 0.13 to 1.3 mum range affect both primary and secondary cracking reactions and the yields of gasoline, coke and light gas yields.