In this paper, we consider control of nonlinear mechanical systems by symmetric controller (DDFB controller). Firstly, we discribe DDFB controller for linear mechanical systems. Secondly, we show that nonlinear mechanical systems are stabilized by DDFB controller. Especially, we also show that for underactuated systems DDFB controller is a class of Euler-Lagrange (EL) controller for underactuated systems and that it is a special class of strict positive real based dynamic output controller to control EL systems. Finally, obitained results are verified by simulations.