Modeling and simulation permeate all areas of business, science and engineering. To promote the interoperability and reusability of simulation applications and link geographically dispersed simulation components, distributed simulation was introduced. While the High Level Architecture (HLA) is the IEEE standard for distributed simulation, a Run Time Infrastructure (RTI) provides the actual implementation of the HLA.With increased size and complexity of simulation applications, large amounts of distributed computational and data resources are required. The Grid provides a flexible, secure and coordinated resource sharing environment which can facilitate distributed simulation execution. In this paper, we propose a Service Oriented HLA RTI (SOHR) framework which provides the functionalities of an RTI as Grid services and enables large scale distributed simulations to be conducted on a heterogeneous Grid environment. The various services in SOHR can be dynamically deployed, discovered and undeployed, leading to a scalable distributed simulation environment. While the communications between simulators are through Grid service invocations, the standard HLA interface is provided as a library to increase simulator reusability and interoperability. A subset of HLA specifications was implemented in a SOHR prototype based on GT4 and the experimental results have verified the feasibility of SOHR.