Abstract: A design principle of propellers was obtained in this paper based on the analysis of the forces. The optimum attack angle principle for the profiles of the propeller was derived from theory of aerodynamics and analysis of mathematics; In the application of this principle the optimum operation condition, 1.e-the maximum power at design wind speed, from stand point of aerodynamics can be guaranteed. A hand calculation method is also described here for a practical example. At the same time time using this principle to 39 profiles in reference ( 3 ) the results of optimum design were shown in figures in this paper with Spline function for the different nodal diameters. A comparision between the results and those of practical unit was made so as to verify the advantage and practicability of this method.