The GridMPI is an implementation of the Message Passing Interface Standard (MPI) designed for high performance computing in the Grid environment. It establishes a synthesized single cluster computer environment by gathering multiple cluster computers from distant locations in the Grid. It realizes latency-aware interoperable communication, and employs optimizations in each level of the communication layers. Since it follows the widely-used MPI standard, users are allowed to develop applications using thier usual environment on a local computer, then to seamlessly deploy them to the Grid.