Numerical simulation of large scale problems has a sharply growing computation demand. One of the solutions of this problem is to apply parallel computing technology. In this paper, the dominant parallel strategy used in engineering application, Domain Decomposition, is discussed and implemented in three parallelization applications. The main focus of this study is a 3-D finite element model, QUODDY, which solves the linearized shallow water equations forced by tidal or other barotropic boundary conditions, wind and/or density gradients. A 2.5 degree turbulent closure model by Mellor Yamada is utilized in the simulation. The sequential QUODDY code is studied first. Then Domain Decomposition approach implemented in the sequential code. By dividing the task into several smaller portions, the computation time was reduced dramatically. Message Passing Interface (MPI) protocols are used for the communications in all processors. Implementation of MPI improves the portability of the paralleled code. The performance of the parallel code is studied and shows good performance. Copyright ASCE 2005.