In recent years the power systems are pushed to operate closer to their limits due to the combination of increased energy consumption and various kinds of obstructions to extension of existing transmission system. A power system is said to be secured when it is free from danger or risk. Security is ability of the system to withstand any one of the pre-selected lists of contingencies without any consequences. Contingencies are potentially harmful disturbances that occur during the steady state operation of a power system. Load flow constitutes the most important study in a power system for planning, operation and expansion. The purpose of load flow study is to compute operating conditions of the power system under steady state. These operating conditions are normally voltage magnitudes and phase angles at different buses, line flow MW and MVAR, real and reactive power supplied by the generators and power loss. A power system which is operating under normal mode may face contingencies such as sudden loss of line or generator, sudden increase or decrease of power demand. These contingencies cause transmission line overloading or bus voltage violations. The contingency selection by calculating the two kinds of performance indices; active performance index and reactive power performance index for single transmission line outage. With the help of Newton Raphson), the Active Power Performance Index (PIP) and Reactive Power Performance Index (PIV) have been calculated in DIgSILENT Power Factory simulation software environment and contingency ranking has been made. During the load flow analysis all the base case, 50% load increment and N-1 line outage has been evaluated and both the pre-contingency and post contingency load and voltage violation is determined and interpreted well. Based on the severity ranking the Bahir Dar- Debre Markos, Bahir Dar- Motta, Bahir Dar- Gondar I and Tana Beles- Bahir Dar lines are the most suffered lines from the existing network. Based on the load flow results and performance indexes, the EEP North-West region should upgrade either the network or construct the reactive power or series compensators on the most risky lines and substations.