Abstract: A method, an apparatus, and a system for selecting a service are disclosed. The method is applied in an Open Service Architecture (OSA), and includes: receiving a service invoking request message sent by an external application; obtaining, according to the service information, policy information corresponding to the service to be invoked; and selecting, according to the policy information, a service provision entity to invoke the service. An open service entity is applied in an OSA and includes a receiving module, an obtaining module, and a selecting module. A policy provision entity is applied in an OSA, and includes a providing module. A service selection system is applied in an OSA and includes an open service entity and a policy provision entity. With the present invention, a proper service is selected for the external application according to the policy, and the services of the communication network are invoked more conveniently and accurately.