Because many scientific applications require large data processing, the importance of parallel I/O has been increasingly recognized. Collective I/O is one of the considerable features of parallel I/O. In this paper we suggest the subgroup method for collective I/O. It is the way of using collective I/Os of MPI effectively in terms of application programs. From the experimental results, we can conclude that the subgroup method for collective I/O is more efficient than plain collective I/O and more reliable than non-collective I/O.