In this paper the effect of generator parameters inaccuracy on the transient stability performance of generator and power system is investigated. Generator parameters are identified either by calculation based on design specification or by measurements. In both approaches, the parameters are obtained with some degree of inaccuracy. Inaccuracy and uncertainty in the obtained parameters can affect the dynamic performance and transient behavior of synchronous generators that may affect transient stability evaluation of power systems. In this paper, using the concept of sensitivity analysis of dynamic performance of generators with respect to their parameters, the effect of inaccuracy of each parameter on the transient stability of power system is investigated and the acceptable and tolerable inaccuracy in the identification of each parameter is evaluated. The proposed concept and approach is demonstrated on the IEEE 39-bus test system.