Fluid flow diverter serves as an essential element in high accuracy liquid flow calibration facility using a static weighing gravimetric system coupled with a flying-start-and-finish mechanism. For such flow system, stability and patterns of flow that passing through the draining nozzle and diverting module influence the accuracy of flow rate measurement. Consequently, the design and implementation of a diverter mechanism with feeding pipe section contribute dearly to the total measurement uncertainty of a standard calibration facility. In this research, a draining nozzle that changes from circular cross section to rectangular was specially developed to provide fluent flow transition. To facilitate the design process, the computational fluid dynamics (CFD) was introduced that effectively reduced the time consumption for prototype construction and test. The selected design exhibited symmetric and stable flow patterns in the desired operational flow range.