In this paper Dc-Decay procedure and its implementation for determining synchronous machine parameters at gas units of Ghom power plant is presented. Dc-Decay time domain data are acquired from standstill test measurements. The direct axis and quadrature-axis dynamic parameters are found through processing the step response of the synchronous machine at standstill position. Genetic algorithm approach is employed to identify the desired parameters. This test procedure reduces the required power and minimizes the testing time; but presents a limited precision because of the low flux level and standstill condition. The simulation results and their comparison to the actual measured signals show the accuracy of the derived models.