Control of open loop unstable process is a challenging one owing to the presence of right hand plane pole(s). The spotlight of this paper is to develop modified Robustness set point Tracking Disturbance rejection and Aggressiveness controller(RTD-A) for an unstable process. The RTD-A controller formulation is modified accordingly to suit the unstable process. The proposed control scheme has two control loops. The unstable process is initially stabilized by an inner feedback loop consisting of a proportional controller. The RTD-A control strategy is then implemented on the stabilized plant. The developed control scheme is validated on an nonlinear unstable bioreactor plant model in a simulation environment. It is found that the proposed controller has better servo and regulatory performance when compared to the conventional IMC-PID controller. Effect of variation in the tuning parameters of the controllers on the process output is analysed in detail. © 2020, IFAC (International Federation of Automatic Control) Hosting by Elsevier Ltd. All rights reserved.