Abstract: A working pressure angle was defined as a function of rotating angle of noncircular gears, called Working-Pressure-Angle or WFA function. A method for generating noncircular gear pairs was thus developed and named. Direct-Profile-Design (DPD) method, where WPA function is used to obtain the desired tooth profiles of generated pairs, was put forward. The length of a tooth profile normal between the tooth profile and pitch curve was defined as the normal length of a tooth profile. This length relates to WPA by a first order differential equation. The differential equation ensures one tooth profile to be engaged with the other profile. As demonstrated, this new method is applicable to any type of gear pairs including circular or noncircular. In addition, using this method is relatively easier to generate new tooth profiles and investigate the geometrical and mechanical properties of gear pairs, and the desired geometrical and mechanical properties will come true in design.