The message-based communication among services in Service Oriented Architecture (SOA) is vulnerable to various security attacks, and has to be well protected by security mechanisms, which may sacrifice service performance due to limited system resources. In this paper, an adaptive tradeoff model for service performance and security in servicebased systems is presented. This model can be used to adjust security configurations of services to provide sufficient protection and satisfy service performance requirements for SOA-based systems simultaneously. The construction of this model includes the development of a set of metrics to quantitatively measure the performance and security of services, the development of a tradeoff objective function incorporating service performance and security, and the parameter estimation through experiments. An example of service-based secure voice communication system is used to illustrate the construction of this model.