Lightning strikes represent a considerable cause of short interruptions in electrical overhead line networks. The over voltages caused by lightning cannot be avoided but their influence can be limited by appropriate over voltage protection. This paper presents analysis study on some factors affecting the back flashover of Egyptian 66 kV distribution lines using Alternative Transient Program (ATP). The study includes the modeling of 66 kV distribution lines, the effects of magnitude and the front and tail times of lightning wave on the back flashover voltages, the effects of the striking distance, and the using of counterpoise wires on the back flashover on the 66 kV line towers. In this paper the transmission lines sag is neglected and the soil ionization is considered.