The on going demand for the improved impeller designs usually comes from the users of industrial mixing equipment. Most of the industrial biodiesel reactors using blade agitator for mixing, which creates a main flow in the reaction tank (reactor) with circulation on axial and tangential direction. In current study, design of mixing system used in STR biodiesel production by two analytical and FEM methods has been made. Designs have been done based on a pitched blade turbine down flow with two inclined blades (45°) by assumption of maximum working conditions (using caster oil as base fluid). The mixing power and the operating limitations of mixing shaft deflection and critical speed during the biodiesel production process by using both methods, has been calculated. The FEM results have displayed close values with the results obtained from the analytical method. The critical rotational speed of the mixing shaft was obtained from the both analytical and modal analysis show similar results 5778.26 and 7000 rpm respectively and found to be significantly higher than the impeller maximum required mixing speed.