Abstract: The yaw control of coaxial helicopters is gained from the differential control of collective pitches of up and down rotors. Hence, a coupling phenomenon can be found. Employing the free wake arithmetic, the aerodynamic analysis for the small coaxial helicopter was made, mainly focusing on the coupling between yaw and collective pitch controls in hover flight. The computation showed the relationship between collective pitches of two rotors when the yaw control or collective pitch control was given. From those results, the characteristics of decoupling control could be identified. And those data was transformed into several simple functions using the curve fitting method. These results will be significant for designing the flight control system.