This thesis presents the design and the stability analysis of a hierarchical controller for unmanned aerial vehicles. We utilize non-linear control methodology to command dynamics of unmanned helicopter which has been divided into slower translational dynamics (outer-loop) and faster orientation dynamics (inner-loop), thus exhibiting hierarchical structure. The attitude angles and position which separately belongs to the inner-loop and the outer-loop can be independently controlled by backstepping control strategy. Stability analysis of the helicopter system including applying dynamics has been illustrated after designing. And relative comparison between linear and nonlinear controllers has been carried out. After studying linear control, nonlinear control and stability analysis of one small-scale helicopter, we think about control strategy of a group helicopter which can be called as consensus control.