To speed up data transmission of data grids, several co-allocation schemes have been proposed. However, data grids are often large in scale, heterogeneous in participating resources, and complicated in architecture and network topology, consequently increasing the analytical complexity of its data transmission behaviour. In other words, if we can reduce the data transmission topology for the grid, the analysis will be easier. Therefore, in this paper, we propose a topology reduction approach, called the Tree-to-Star Reduction method (TSR for short), which can reduce a packet delivery tree topology to a star for a data grid so that the data transmission of a co-allocation scheme can be more conveniently analyzed. Here, a delivery tree topology, as a tree topology rooted at the destination node, is a network topology for delivering all fragments of a file to the destination node.