A one-dimensional adiabatic mathematical model for the riser reactor of an industrial fluid catalytic cracking unit, FCCU was presented. The cracking reactions in the riser reactor were based on six-lump kinetics of the catalytic cracking of vacuum gas oil, taking cognisance of diffusion resistance, which is a departure from the general norm in the literature. The developed model was a set of eleven highly non-linear, coupled and stiff ordinary differential equations, ODEs, which was numerically solved with an implicit MATLAB built-in solver, ode23tb, designed deliberately for handling stiff differential equations to circumvent the problem of instability associated with explicit methods. The industrial plant data of China National Petroleum Corporation (CNPC) were used to validate the simulated results of this study. Excellent agreements were achieved between the CNPC FCCU plant data and the simulated results of this study for instantaneous vaporization of feedstock, with AAD being < 5% in all cases investigated, where the optimal yields of the cracked products needed to meet market demands and ensuring maximum profit were achieved.