This paper presents the design of the lightweight communication interface called RWAPI over the Infini- Band interconnect for clusters of PCs. RWAPI has been developed to provide performance to higher applications on a wide variety of architectures. Since the specifications of the InfiniBand interconnect provides many ways to transfer data, we are discussing some issues regarding the choices between InfiniBand capabilities. We implemented RWAPI using the grid-oriented architecture called GRWA and evaluated the communication performance. We obtained a very low latency and a throughtput very close to the maximum user bandwidth for messages as small as 4 kilo-bytes.