As NOW (networks of workstations) is increasingly used to run scientific applications, how to provide high performance I/O for I/O-intensive applications has become one of the crucial components in NOW environments. In the paper, the design and implementation of Collective I/O system on NOW is presented. The system combines the advantages of both DDIO (disk-directed I/O) and two-phase I/O and a series of optimizations such as data sieving are employed. The initial performance evaluations have shown good I/O performance.