With the increased demand for developing renewable energy, hydro energy has attracted more attention since it is reliable and easy to acquire. In this area, the cycloidal turbine has been recently studied and applied to ocean energy for its stable and efficient output. Compared to the ordinary vertical/horizontal axis turbine with fixed pitch angle blades (e.g., Darrieus turbine), the cycloidal turbine can maximize the extracted power efficiency by keeping the optimized angle of attack for the blades. Meanwhile, the cycloidal turbine provides a potential solution to solve the problems of self-starting and seasonal flow variations. Introducing an augmentation duct is considered as a method to further increase the incoming flow velocity of the turbine. Inspired by the design of the wind tunnel, a convergent-divergent design of the augmentation duct is developed. One is noted that the dimensions of the augmentation duct are essential to the performance of the duct. In this study, a convergent-divergent augmentation duct is developed based on a 3-blade cycloidal hydro-turbine, operated at a 2 m/s river. Computational fluid dynamic (CFD) analysis with sliding unstructured mesh is applied to investigate the extent how the dimensions of the duct affect the flow velocity to the turbine as well as the extracted power efficiency.