In a visualization system, one of the key issues is to optimize performance and visual fidelity. This is especially critical for large virtual environments where the models do not fit into the memory. In this paper, we present a novel structure called HDoV-tree that can be tuned to provide excellent visual fidelity and performance based on the degree of visibility of objects. HDoV-tree also exploits internal levelof-details (LoDs) that represent a collection of objects in a coarser form. We also propose three storage structures for the HDoV-tree. We implemented HDoV-tree in a prototype walkthrough system called VISUAL. We have evaluated the HDoV-tree on visibility queries, and also compared the performance of VISUAL against REVIEW, a walkthrough system based on R-tree. Our results show that the HDoV-tree is an efficient structure. Moreover, VISUAL can lead to high frame rates without compromising visual fidelity.