As the number of nodes in high performance computing (HPC) systems increases, parallel I/O becomes an important issue: collective I/O is the specialized parallel I/O that provides the function of single file based parallel I/O. Collective I/O in most message passing interface (MPI) libraries follows a two phase I/O scheme in which the particular processes, namely I/O aggregators, perform important roles by engaging the communications and I/O operations. In this paper we study the effects of the collective buffer that is used by I/O aggregators during data exchange phase and I/O phase. The result of preliminary experiment shows that the collective buffer size affects the collective I/O performance and the efficient collective buffer size should be defined by considering multiple factors such as data distribution rules, file system configuration and so on.