Penetration of a large number of wind turbines into existing electric power systems can bring complexities in system stability, reliability evaluation and power management. This can lead to potential system problems during various disturbances. Transmission system operators should analyze the impact to the system parameters, and problems related to ancillary services and system stability. Analyses show the factors which affecting transient stability. The influence of the variations of rotary inertia on critical clearing time of power system transient stability is analyzed by time-domain simulation. We have study the impact of wind power plant on Albanian Power System transient stability. Simulations show that by replacing synchronous generators with wind power unit, the overall system inertia is decreased which lead to reduction of critical clearing time value. The difference between the critical clearing time in some cases is reduced up to 15% of the base case value.