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Path tracking of Unmanned Aerial Vehicles (UAVs) with three degrees of freedom is studied in this paper with approach of dynamic sliding mode control. For this purpose, the equations of UAV are written. The difficulty and complexity of these equations is that they are non-affine with respect to control inputs. Moreover, they are not directly in the inertial coordinate system while the desired flight path is given in the inertial coordinate system. These two major problems add complexity to the design procedure. Therefore, it is necessary that equations be rewritten in the inertial coordinate system. By definition of virtual inputs; the equations convert to affine structure with respect to virtual inputs and the transformation between the real and virtual inputs has been obtained. After that, the Input/output (I-O) equations of the system are written and converted into controller canonical form. The dynamic sliding mode control law is then designed based on (I-O) equations. Optimal coefficients are also achieved numerically by considering an appropriate cost function. Finally, computer simulation is utilized to illustrate the performance of the designed controller.