This paper is concerned with the problem of detecting and isolating multiple faults by a full-order observer of the plant state. We first obtain the fault isolability condition from the dual concept, namely static-state feedback decoupling, A new full-state observer design method is then developed to isolate simultaneous multiple faults. The method tan isolate m simultaneous faults with nz output measurements (such that the isolability condition is satisfied) under zero initial error of state estimation. Asymptotic fault isolation results can also be obtained using the same observer design if the number of invariant eigenvalues is (n - m), and if they are stable, where n represents the system order, m is the number of faults in the system, In both cases In fan be up to the system order n.