A new approach to design a nonlinear observer for the Wind Energy Conversion System (WECSs) with the existence of a delay time state variable is proposed in this paper. The WECSs are modeled under a polynomial framework that allows us to apply the powerful methodology of the linear system to synthesize the observer. Unlike the previous paper, the delay time in this paper is arbitrary and unnecessary to satisfy the bound constraints or the time delay must be constant. Additionally, during operating the WECSs, the wind is taken into account the unknown disturbance that affects to the performance of WECSs. In this paper, the unknown input method is employed to design the observer which can eliminate the influences of the delay time parts and disturbance; as well as estimate the unknown states asymptotically. Based on the Lyapunov theory and Sum-Of-Square (SOS) technique, the conditions to synthesize the nonlinear observer are obtained in the main theorems. The simulation results are also provided to show the merit of the proposed method.