Abstarct: Abstract Since transmission lines are always exposed to various faults, fast and accurate fault location plays an important role in improving reliability, stability and outage reduction of power networks. On the other hand, today, the use of multi-terminal lines are developed due to economic and environmental benefits. Among multi-terminal lines, three-terminal lines have the most popular configuration. The protection of these lines is more complicated than the two-terminal lines due to the impact of the third terminal fault current on the voltage and current of the two main terminals. Most of the fault location methods use data from phasor measurement units (PMUs) installed at all terminals, which increases the cost of installation and maintenance of these equipment. It is worth noting that, some of the methods that perform fault location by reducing the number of phasor measurement units have problems such as assuming a constant pre-fault and post-fault Thevenin voltage of the terminal without PMU or not detecting the fault section in the areas of three-terminal lines. Therefore, in order to overcome these drawbacks, in this thesis, an impedance-baxsed method for fault location in three-terminal transmission lines by reducing the number of phasor measurement units is presented. The proposed method is able to determine the fault section and location simultaneously. The simulation results indicate the optimal efficiency of the proposed method in different conditions, so that the accuracy of the method is generally less than the standard range. Morover, the accuracy of the proposed method is not affected by the occurrence of faults close to T-off point, the various of fault inception angle, the inaccuracies caused by measurement errors and data synchronization, the change of the source impedance and the change of the power flow direction.