A simple genetic algorithm is applied for tuning of PID controllers for the cascade control systems. A methodology for selecting the search region is proposed using Ziegler-Nichols tuning method. Stability and robustness criteria are ensured in the selection of the search region, enabling the method to be applicable to online tuning. The inner and outer loops are tuned simultaneously, making the method applicable without disturbing the control strategy and ensuring overall optimal solution. The sum of integral absolute error values of the regulatory response is used as the objective function. The performance of this method is compared with the random search method of Lee et al. and Luus and Jaakola (LJ) algorithm. The results show the superiority of genetic algorithm (GA) over the other methods. The effect of unstable domain in the region of search is studied for both GA and LJ. © Indian Institute of Science.