Global demand growth for electric energy has speed up development in designing power systems in order to meet users' needs for providing reliable, cheap and high quality electric energy. In power networks the first zone of distance relay is considered as main protection for transmission lines. Zones 2 and 3 of this relay are considered as supporting protection for next lines. Distance relay can be affected by many factors which are not clear and definite. These factors which are considered as uncertainties are fault resistance, fault in measurement transformers, ground resistance, and change in structure and exploitation conditions. In this article we prove their considering uncertainties approaches zone 1 distance relay extent to 100 percent of protected line provides more reliable protection for transmission lines.