This paper presents EDF schedulability driven security optimization in real-time systems. An increasing number of real-time applications like aircraft control and medical electronics systems require high quality of security to assure confidentiality and integrity of information. However, security requirements were not adequately considered in most existing real-time systems. We propose a group based security service model for real-time systems where the services are partitioned into groups. Services in the same security group provide the same type of security service but of different quality due to the different mechanism used. Service from different groups can be combined to achieve better security. The overhead model of the security services is also described. We consider EDF scheduling policy and develop a security aware EDF schedulability test. Two approaches: Integer Linear Programming technique and an efficient heuristic search technique are proposed to select the best combination of security services for real-time systems while guaranteeing their schedulability.