This paper presents the design of a PD + I fuzzy controller applied to control the temperature of a nonlinear chemical process with variable dead time which is composed of a mixing tank. One of the biggest problems when fuzzy controllers are designed is the definition of its membership functions and gains (in case of these exist). For tuning and getting the best parameters of the controller, the optimization is performed using the evolutionary algorithm PSO (Particles Swarm Optimization), which needs minimize an objective function. In this work, it has been chosen the Integral Square Error (ISE), performance metric that allows knowing how fast the controller takes the system output to the desired reference. Finally, the proposed optimized controller is compared by simulations with other controllers applied to a mixing tank with variable dead time.