Providing online services for consumers of electricity is the main concern of distribution companies. Consumers’ time off is one of the effective parameters on distribution systems’ reliability. Thus time off should be decreased heavily to provides better services for consumers. For this purpose, it is essential to identify where the fault occurred quickly. A reasonable estimation for a fault location can help the solution of the problems in less time, reducing the interruption time in the energy supply, especially for permanent faults. For temporary faults, an accurate fault location is needed to prevent future permanent failures that could bring more damage to equipment and unsafe situations to public. Most studies were about the fault location in transmission lines and distribution systems were less considered. In recent years, by the development of distribution automation systems, more researches have been done on the fault location in distribution systems. One of the most important fault locating methods is apparent impedance algorithm in the distribution systems. This algorithm uses the fundamental component of voltage and current to detect the fault location. The apparent impedance algorithm was based on fixed admittance load model. this means that, the apparent impedance algorithm are dependent on the system loading during the fault period, as one of the required inputs is the load in each node of the system. Whereas, the load demand in distribution systems is depended on the consumer type (residential, industrial, or commercial) and may vary with many factors. Different load profiles during the algorithms execution, will lead to errors. If the apparent impedance algorithm is been corrected, load changes in distribution system and various short circuit types will not affect the algorithm’s precision. In this thesis, the apparent impedance algorithm has been simulated for all work’s states and faults. Results of the simulations illustrate acceptable accuracy in predicting the fault location by this method. Distribution systems were normally radial systems with unbalance, in other words, it used to have only one direction of power flow and different loading at the laterals. But nowadays, several technologies in development walk through different kinds of distributed generation. Distributed generation changes the system's radial characteristic since it’s a new source of power in the system. Therefore, presence of distributed generation lead to occurred large errors in determining the fault location by the apparent impedance algorithm. In this thesis an attempt is made to propose a new algorithm, which is the modified of the original... Keywords: Unbalanced Distribution System, Fault Location , Distributed Generation, Power System Protection