Testing has become an important design step now-a-days in digital circuit. A gate level fault simulation environment based on realistic fault models has been presented in this paper. A Genetic Algorithm (GA) is proposed which allows having fault simulation with conditional execution of test vector under 2 phase scheme. By using this approach, a random search of test vectors is possible without being caught in a local minima or maxima. The award of fitness to the vector set allows having a selection of test vectors with high fault coverage and with large fault detection scores.