In this paper, a robust filter for an in-flight alignment is presented to effectively eliminate system errors in the case where a strapdown inertial navigation system(SDINS) has large initial attitude errors. First, an extended robust H-inf filter is proposed for a general nonlinear uncertain system. We also analyze the characteristics of the proposed filter such as an H-inf performance criterion using the Lyapunov function method. Analysis results show that the proposed filter has a robustness against disturbances such as process and measurement noises and against parameter uncertainties. Then the in-flight alignment for the SDINS is designed using the presented filter. Simulation results demonstrate that the proposed filter effectively improve the performance