The main objective of this paper is to present a study about the identification and reduction of harmonic distortion in an uninterruptible power supply system of a petrochemical industry, located in southern Brazil. Harmonic distortions were measured and identified, and then the system was modeled to reproduce its real behavior in the simulator software. In order to reduce the harmonic effects, a passive tuned filter set was proposed. Simulations present a current harmonic distortion reduction in the order of 70%, from the initials 50.4% to 15%, reaching international recommended limits. In addition, it was observed load power factor improvement and a reduction in the current peak value. The results found so far are promising to solve existing harmonic inconvenience in this uninterruptible power supply system that attends to critical process equipment.