Abstract: The reliability of the strap-down inertia navigation is discussed and the parity equations are formulated by using parity vectors.The hypothesis approach is used to establish the decision function and isolation function for failure detection.This paper also studies the Kalman filtering method to obtain estimations of dynamic error and other errors which are later used to compensate the parity vectors,and implements the failure detection and failure isolation by employing a constant-value threshold.Simulation results from a strap-down redundant system of 6 single-DOF gyroscopes demonstrate the effectiveness of this method.