A parallel volume rendering system that incorporated a hardware compositor was developed to visualize unsteady large-scale data set in real-time. This rendering system is constructed by PC-cluster on a master-slave basis, and utilizes hardware functions of GPU on a modern graphics card. At first, a master node divides a data region into several sub-domains, and then each slave node renders the images for each domain. Generated images on each Slave-PC are gathered and overlapped each other according to a BSP tree, which is determined by the relation between an eye point and the position of each domain. This superimpose process is carried out on a composite hardware to minimize the transmission and synthesis time of the images. It was found that the present visualization system achieved enough interactivity with 10-15 fps for time varying data of 25.6GB.