: Nowadays, power quality (PQ) plays an important role in distribution systems. Complexity and sensitivity of electric devices are strongly increasing so that changing of waveform amplitude, frequency and distortion can cause a heavy damage on equipment and consumers. So, it’s necessary to measure the PQ parameters and determine pollution and their disadvantages in the network. On the other hand, use of distributed generation (DG) is one of the basic methods of energy generation in distribution networks. In order to use DGs, considerations of network parameters like PQ are very important because the use of these resources affect the PQ of system. This thesis is aimed at studying the reasons of PQ problem like voltage sag swell, harmonics, flickers and voltage imbalance, together with considering the effect of DGs on them. Therefore, pollutant loads are used and their effects are considered. In order to simulate voltage sag, all kinds of faults like three phase, two phase and single phase to ground are applied. Also, induction motor starting and capacitors switching are used for voltage sag modeling. Twelve pulse inverter and adjustable speed drive are applied as harmonic sources for harmonic simulation. Arc furnace is used for modeling flickers. In order to simulate arc furnace, voltage-current model is used that simulate it as real one. Then the effects of all faults and mentioned loads are considered in the network without presence of DGs. In order to study the presence of DG and its effect on PQ parameters, three kinds of DGs called synchronous generators, induction generators and inverter based DGs are used. In this regard, control methods are used to mitigate or decrease PQ problems with respect to voltage and current quality and harmonic levels improvement. So, deployment of inverter based DG with Z-source inverter, simple and advanced control method, voltage detection with Hilbert transformation are considered for voltage sag mitigation , harmonic, flicker and unbalanced voltage compensation, respectively. As a matter of fact, especial methods are used for sag mitigation in inverter based DGs. As mentioned, using series compensation for inverter based DGs for decreasing voltage sag shows a very good compensation in the system. Also methods for harmonic mitigation works very well in real distribution systems. Using these methods causes mitigating the injecting current of inverter based DGs without any harmonics. The EMTDC/PSCAD software is used for simulation program. 13 buses IEEE test feeders is selected for case study because it has all kind of loads like power , impedance and current constant and can be used as an small real network. Keywords: Distribution network, Distributed generation, Power quality.