The use of diffuser on the hydrokinetic turbines improves its efficiency, exceeding the Betz limit (59.26%). The diffusers are technologies which has the function of causing an effect of increase in flow velocity that arrives on the turbine blades. This effect is caused by acceleration of the fluid particles due to the pressure drop downstream of the diffuser. In this paper, one describes a numerical study of the flow around three different geometries diffusers applied to the horizontal axis hydrokinetic turbine design. We evaluate the behavior of the diffuser velocity speed-up ratio in order of ranking the efficiencies of three different geometries. The numerical study is performed using computational fluid dynamic (CFD). The numerical model is validated using experimental data available in the literature. The results are applied to the case of a hydrokinetic turbine and the horizontal axis shows were satisfactory.