Abstract: Conventional fault diagnosis technique based on adaptive observer can not be used for non-minimum phase systems.Using reduced-order observer,this paper proposes a new adaptive fault estimation method.Firstly,special co-ordinate basis(SCB) transformation is introduced to calculate invariant zeros of the error systems conveniently.Then,a novel fast fault estimation method based on reduced-order observer is proposed for the changed system with the purpose of improving the fault estimation performance,i.e.,rapidity and accuracy of fault estimation.Finally,simulation results are presented to illustrate the efficiency of the proposed method.