In this paper, a control law is presented to regulate yaw angle of an unmanned helicopter. The proposed law has been designed to control yaw angle at turning manoeuvre, where the helicopter centroid motion and pitch rotational speed are almost zero. Governing differential equations of the helicopter at turning manoeuvre are nonlinear and coupled. Feedback linearization was employed to compensate the nonlinear part of system dynamics, and a proportional derivative (PD) controller was designed to improve the response. The designed control law was tested in realistic simulations considering reasonable/implementable noises, disturabnces, delays and sampling frequencies. The results demonstrate excellent performance of the proposed control law.