An important issue in the design of a mobile computing system is how to manage the real-time locations of mobile clients. In the existing commercial cellular mobile computing systems, a two-tier architecture is used . However, the two-tier architecture is not scalable and is not suitable to the new mobile computing applications. In the literatures [1, 12], a hierarchical database structure is proposed in which the location information of the mobile clients within a cell is managed by the location database responsible for the cell. The location databases of different cells are organized into a tree structure to facilitate the search of mobile clients. Although this architecture can distribute the update and searching workload amongst the location databases in the system, it has the problem of heavy location update overhead and long search delay. Thus, it is not suitable to the system which is supporting real-time queries. In this paper, we study how to generate location update in the distance-based update method. Two methods are proposed for calculating the optimal distance threshold with the objectives to minimize the total location management cost and the latency in searching the location of a mobile client.