Abstract: This paper proposes aUIO-based fault detection approach to the actuator of tilting control system of tilting trains. The approach is the use of the disturbance de-coupling principle, in which the residual is designed to be insensitive to unknown disturbances, whilst sensitive to faults. This is done by using a full- order unknown input observer (UIO). The residual can be used to detect faults according to a simple threshold logic. Simulation results showthe efficiency of this method.