In this paper we will propose and design an all optical amplitude shift keying demodulator using a photonic crystal based nonlinear ring resonator. The proposed structure will be designed such that it can generate digital 0 and 1 based on the amplitude of the input signal. The working mechanism is based on controlling the optical behavior of the resonant ring using optical intensity. The proposed structure was simulated using plane wave expansion and finite difference time domain methods. The simulation results show that the rise and fall times for the proposed structure are 0.5 and 0.1 ps, respectively.