Local Area Networks (LANs) are important for the enterprise to hold competitive edges. Therefore, many companies have converted terminal-based computing systems to LAN-based distributed data processing systems. In this case, how to design the distributed databases is an important issue. This thesis develops a decision support system for designing disributed databases (for short, $DSSD^3$) connected by a LAN. The two primary objectives of $DSSD^3$ are (ⅰ) to allocate data files and workload among heterogeneous servers and (ⅱ) to decide the number of servers to satisfy the response time for processing each transaction. The file and workload allocation problem is formulated as a nonlinear zero-one integer programming problem. This problems is proven to be NP-complete. Thus a heuristic is developed to solve this problem effectively. A decision support system is implemented and an example is solved to illustrate the practical usefulness of the system.