Computational fluid dynamic (CFD) technique has been used to investigate the effect of the diffuser expansion angle on the power coefficient of a hydrokinetic turbine. The performance coefficient was compared to that of a conventional bare hydrokinetic turbine of 1 Hp (746 W) designed for a water velocity of 1.5 m s−1 with a tip speed ratio of 6.325; an attack and pitch angle of 5 and 0 degrees, respectively; a power coefficient of 0.4382; a drive train efficiency of 70% and a S822 airfoil profile. The CFD results showed that the extracted power of the hydrokinetic turbine with diffuser can be larger than the power extracted by a bare turbine with the same area. Nevertheless, the level of stress on the blade structure was also increased due to the hydrokinetic turbine with diffuser was exposed to a larger structural loading condition by the water flow.Additionally, the numerical analysis of a damper located at the exit of the turbine diffuser was developed. It was observed that when the opening angle of the damper is between 0 and 20 degrees, an increase in the mass flow through the turbine blades is achieved, resulting in a slight increase in the power coefficient. © 2017 American Institute of Chemical Engineers Environ Prog, 37: 824–831, 2018