This research concentrates on the impact of diffuser and nozzle angles as changed parameters on flow velocity at diffuser passage. Diffuser length, nozzle length, throat diameter are fixed and diffuser and nozzle angles are varied 5̊ to 30̊ and 5̊ to 15̊ while the flow field analysis has been carried out using commercial software ANSYS CFX .The simulation results also show that diffuser and nozzle angles at (30,15) are the optimum angles that accelerates flow to the turbine. The velocity at these optimum angles are higher than other angles which cause the turbine can generate more power output. After getting the optimal inlet-nozzle diffuser design, numerical analysis for the designed hydrokinetic axial flow turbine with inlet-nozzle diffuser have been performed by using ANSYS CFX. This study also focuses on the flow velocity and pressure variation around the turbine due to the effect of inlet nozzle diffuser. Hence, the installation of diffuser around the conventional turbine significantly increases its power output capabilities. The proposed turbine design is intended to use at the irrigation channel so that turbine diameter and flow velocity is considered based on the selected location. According to the simulation results, incoming velocity 1.50 m/s at the throat of the turbine was increased up to 4.03 m/s and the preliminary designed turbine power output 210 W was increased to 288 W by installing inlet-nozzle diffuser to the hydrokinetic axial flow turbine.