Simulation is one of the most frequently used techniques for production system study and design. Simulation created by an object-oriented program has the advantages of hierarchical structures, inheritance, polymorphism and object re-use. This study uses eM-plant, the standard object-oriented software to model the order picking system. Order picking is an essential operation and the major cost constituent of the distribution center. This study attempts to provide the warehouse manager with order picking and operation; examines several storage assignment policies and several sensitivity analyses. The simulation experiment results can offer reference bases for warehouse administrator decision-making.