Users or customers of the electrical power systems must pay attention to some factors during operation and design of these systems in order to operate safe and in desired format. One of these factors is harmonic caused by non-linear elements determining the quality of the power as a parameter. Some harmful effects of harmonics can be listed as; disruption of control devices, explode of capacitors, overload of transformations and additional losses in rotating machines, motors, and the occurrence of extra noise to motors and other elements, to cause telephone interference, parallel and serial resonance frequency. The most important and useful method of eliminating or reducing of harmonics is using harmonic filters. In this study, two different factory in the city of Van located at the east of the Turkey are examined in detail with a variety of different dates by Zera MT310's power analyzer measurements (instantaneous electrical values, harmonics, current and voltage waveforms, etc.). With these measurements, some power lacks of quality are determined and examined. After this process, single-tuned passive filter is designed by using Matlab/Simulink software for these factories.