Abstract: Applications with large amounts of data bring the mode of out-of-core computation in which I/O becomes the important limiting factor because of the low speed of accessing data on disks. A method of using runtime library is presented for I/O optimizations. Three optimization strategies including data sieving on regular section, data prefetching and data reuse on the edge are described. Programmers may adopt corresponding APIs for different applications to reduce the execution time. The experiment results show that the performance of the out-of-core computation is efficiently improved by reducing the number of I/O operations and the amount of exchanged data between the main memory and disks as well as hiding part of the I/O operation latency.