This paper introduces an adaptive fractional order sliding mode control (AFOSMC) for controlling a nonlinear system. The proposed AFOSMC consists of two units. The first is the fractional sliding mode control unit that incorporates the features of fractional order control and sliding mode control. The second unit is the tuning mechanism, which uses a fuzzy logic system for online adjusting the parameters of fractional sliding mode controller. Tuning the parameters provides an extra degree of flexibility in controller design and improves the control performance. In addition, the stability analysis of the proposed controller is studied using Lyapunov theorem. To investigate the effectiveness of the developed controller, it is applied to a van der pol oscillator system. Simulation results indicate the improvements of the developed controller under external disturbance and variation of desired output.