Data Grids are an emerging technology for managing large amounts of distributed data. This technology is highly anticipated by scientific communities, such as in the area of astronomy and high-energy physics, because their experiments generate massive amounts of data which need to be shared and analysed. Since it is not feasible to test different usages on real testbeds, it is easier to use simulations as a means of studying complex scenarios. This paper presents our work on incorporating data Grids features as an extension to GridSim, a computational Grid simulator. The extension provides essential building blocks for simulating various data Grids scenarios. Moreover, it is designed to be easily extended. This approach makes it easy to try various strategies and to add functionalities to suit the needs of other communities. This paper also gives a detailed description of the design and usage examples demonstrating the versatility of this tool. Copyright © 2008 John Wiley & Sons, Ltd.