This paper deals with the design of unknown input observers for nonlinear systems. An algorithm which determines if the state and the unknown input of the system can be recovered in finite time is introduced. This algorithm leads to the transformation of the system into an extended block triangular observable form suitable for the design of finite time observers. The proposed method is useful to relax some restrictive conditions of the existing nonlinear unknown input observer design procedures. (C) 2009 Elsevier Ltd. All rights reserved.