Power loss is the function of power system structure, load type, generation and also different voltage levels that a system is utilized under them and there is no accurate method for determining it.Until now Most of the researches in this field are theoretical so that for determining the power loss with theoretical approach only mathematical equations can be used and in these equations inrush currents, leakage and some environmental conditions do not exist and the effect of harmonics on cables' impedances and conductors will be determined from theoretical point of view and so the calculations are not accurate and real.In this book in most of the calculations, impedances are presumed to be linear and so for High Voltage Power Line Loss estimations, a 14-bus IEEE neural network model is used. After estimating the loss, by the use of measurement devices on the sample lines the real power loss will be determined then the accurate estimations can be done for other lines with different environmental and technical conditions żeby the use of neural networks.In this book the neural networks results are compared with the theoretical results gained from mathematical models and finally