In the present research, the effects of dielectric barrier discharge plasma actuators on the flow control behind a backward facing step with various step angles are analyzed. A multi-objective optimization study has been conducted by means of evolutionary algorithms assisted by surrogate modelling coupled with computational fluid dynamics. A set of decision variables including input voltage, frequency, width of generated plasma and distance from the flow inlet to the start of the generated plasma are employed for an optimization problem aiming to simultaneously minimize the total pressure loss and reattachment length, while maximizing the uniformity of the flow. The flowfields have been analyzed for selected individuals from the Pareto optimal front in comparison with the baseline and reference results in the absence of a plasma actuator. Global sensitivity analysis has been performed to identify key design parameters for flow control. It has revealed major impact of the design parameters of the DBD plasma actuator on the behavior of the flow and major improvements in performance for all three angle configurations. Flow separation has been suppressed considerably while achieving moderate improvements in the flow uniformity and total pressure loss.