In this paper, an optimal proportional–integral–derivative controller (PID) controller is designed for level control of three tank system. The integral-square-error (ISE) based approach is used for tuning of PID controller. The ISE of unit step response is minimized for obtaining optimal controller settings. The ISE is constructed in terms of alpha and beta parameters. The teacher learner based optimization (TLBO) technique is used for minimizing the ISE. The TLBO is recently proposed optimization algorithm which is free from algorithm-specific parameters. The results obtained using proposed technique is compared with other well-known methods. It is found that the proposed technique performs much better when compared to others.