This paper, proposed a hybrid strategy for nonlinear control of six degree of freedom (6 DOF) underactuated trirotor Unmanned Aerial Vehicle (UAV). The hybrid control strategy consists of Nonlinear Disturbance Observer (NDO) along with Proportional, Integral and Derivative (PID) controller to control the model of nonlinear (6 DOF) underactuated trirotor UAV. The control model of the UAV is divided in to two submodels, altitude control and the attitude control such that, the PD controller is

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... to control the altitude of the UAV and (NDO)with the PID controller is used to control the attitude of UAV. However, the stability of the aircraft is proved by using Lyapunov stability criteria. The robustness of the proposed control strategy is compared with the nonlinear observer design with backstepping control of UAV. It shows that the proposed hybrid strategy have better response less steady state error and good robustness in the presence of continuous disturbance in the model of UAV. The nonlinear functions 0 ( ) and ℎ 0 ( ) that can be written as,