Fluid catalytic cracking is a complex interactive process in which heavy distillates like gasoil is converted to more valuable products such as gasoline, diesel etc.,. FCC process is known to be very difficult to model and control because of large process scale and complex kinetics of cracking and coke burning reactions. In this paper, the mathematical model of reactor and regenerator based on the energy balance has been derived and optimal controller for highly non-linear, inter dependant FCCU process has been designed. The performance of FCCU with the decentralized PID control scheme is compared with the optimal control scheme using MATLAB/SIMULINK. The result of simulation discloses the effectiveness of the optimal control scheme over decentralized PID control scheme.