Given the direct relationship between distribution networks and consumers of the power systems, the reliability of these systems is highly important. So improvement of reliability is the top priority of distribution networks designers and operators. One of the solutions provided in this area is the installation of distributed generation in distribution networks, which allows the islanded operation of the distribution system at the time of fault in upstream networks. In this project, a detailed analytical model was proposed to evaluate the reliability of distribution networks considering distributed generation. In order to more accurately modelling, the time required to recover from a main source, the time needed to open and close the switches and the time needed to recover loads through an alternative source were considered. Moreover, the age of distributed generation sources was modeled and its impact on reliability was examined. Also, the time and volume of calculations were reduced using the concept of segmentation and calculation of reliability indices for each segmrnt. The proposed model was used to evaluate the reliability of a sample distribution network. The results indicated that as the age of distributed generation increased, the reliability of the distribution system was reduced.Keyword -reliability of distributed systems, distributed generation (DG), analytical evaluation, islanded operation. I. INTRODUCTIONIn recent years, many efforts have been made to improve the reliability of power systems and distribution networks. One of the solutions considered by many researchers to improve reliability is the implementation and use of renewable energy resources in power systems. In order to improve the conditions for the use of distributed generation (DG), a new concept called microgid was introduced that enables the integration of DG sources and more efficient use of them [1]. Given the increasing presence of DG and microgrids in distribution systems, evaluating the reliability of these networks has become more complex. The evaluation of the reliability of distributed networks along with DG has attracted the attention of many scholars. In some studies, the reliability of microgrids with distributed energy sources such as wind and solar energy is evaluated. For example, in [2,3,4,5], the reliability of distributed networks along with DG is evaluated. In [6], the impact of DG and storage systems on the reliability of the distribution system is studied. In this study, storage system modeling was used to reduce the fluctuations in output power of renewable energy resources.There are also studies to use Monte Carlo Simulation (MCS) to evaluate the reliability of the system. In [7,8], the reliability of small islanded power systems and independent microgrids is calculated using load simulation and MCS. In [9], a comprehensive review of reliability models and methods for estimating the impact of renewable energy resources on the availability of generation power is provided. In [10], a study on evaluating the reliab...