Harmonics generally occur due to the presence of non-linear circuit elements and / or nonsinusoidal sources is found in the power system.Harmonics are pollution in power systems. With the use of static converters, this pollution rate increases day by day. The increase of power electronic elements and various nonlinear elements every day causes the increase of the non-sinusoidal size circulating in the energy system. The presence of harmonic components in power systems will lead to distortion of the sinusoidal wave.As a result of non-linear elements or non-sinusoidal sources, the sinusoidal wave form in the energy system is distorted. These distorted waves are called "non-sinusoidal waves". The wave outside the fundamental wave is called the "harmonic component".Demand for electricity energy is increasing day by day. Harmonics must be destroyed for a more reliable and higher quality energy.Considering increase of non-linear elements in recent years, it appears that harmonics will greatly affect our energy system in the near future. Harmonic currents tend to flow from the harmonic source to the lowest impedance in power system.Harmonics producing by non-linear characteristic loads or sources are added to power system. This paper analyzing variousnon-sinusoidal voltage or current distortion waveforms generated by typical non-linear loads such as uninterruptible power supply (UPS), DC/DC power converter, frequency converter, DC/AC rectifier, switched power supplies, saturated magnetic components and power electronic switches. Even if the power of the nonlinear loads are low, they distort the sinusoidal wave form of current or voltage. Effects of harmonics on power system can cause such problems as change of power factor, neutral current increase, heating of transformers, increase of losses, capacitor fuse blowing,Increase in voltage drop in power system, memory of CAD / CAM terminals deletedvibrations in electric motors, resonances and device malfunctions. Nonlinear loads, even at low power, distort sinusoidal current and voltage waveforms in energy systems. Considering the large number of nonlinear loads connected to the power systems, it is very important for the quality of the energy to eliminate the resulting harmonic distortion. Nonlinear elements cause serious harmonic pollution in production, transmission and distribution systems and decrease the quality of energy given to consumers, and harmonics in power system must be destroyed before larger problems occur. This paper presents a detailed analysis of total harmonic distortion (THD) in terms of presence of a nonlinear load in the power circuit. The six pulse rectifier in the power system act as a harmonic source. The harmonic components such as the 5th, 7th, 11th, 13th, 23rd, 25th, etc. produced by a six-pulse rectifier.Simulink is a bundle program of Matlab that is used to build, simulate and analyze dynamic system models. The hamonic analysis of the power system was performed with the help of the Matlab /Simulink program.