Real time systems (RTS) operate in an unpredictable changing environment that causing certain system performance features to degrade. Such systems need robustness to guarantee limited degradation despite variations in environmental parameters. EDF (Earliest Deadline First) scheduling has been used to evaluate the robustness of uniprocessor system with makespan as performance parameter. Robustness of real time system is directly proportional to the makespan of the resource allocation.EDF produces the optimal schedule that maximizes the robustness in the system.