Friction in the electro-hydraulic servo system makes tracking and pointing accuracy lower, it is one of the most important influence factors. This paper proposes a nonlinear adaptive friction compensation method based on analysis of the position-control system of valve-control asymmetrical hydraulic cylinder and the LuGre friction model. Dual nonlinear observers were constructed to estimate the friction state, which is unmeasurable in the electro-hydraulic servo system, and adaptive laws of friction parameters were built to compensate dynamic friction. Finally the application of Simulink has proven this method is validity.