The purpose of this study is to define the reliability index for electrical power systems specifically and without using alot of data and treating processes. Results of this study is useful in electrical networks planning and design because of its technical and economical importance in the optimal choosing of networks elements and connections. In this research we use petri nets concept to represent electrical nets under studying then simulate the equivalent chart on GRIF program to find reliability degree for the system and its elements. This research includes comparing the suggested methodology to a reference net and then apply this method on electrical local network to discuss the results. It has been proved that petri nets are effective in reliability calculations with the best of simulation methods.