This paper is concerned with the observer design problem for a class of multi-input-multi-output nonlinear systems with the unmodeled dynamics, unknown parameters and external disturbance. A dynamic signal, which can dominate the unmodeled dynamics, is firstly constructed. Then, two types of observer schemes, that is, adaptive observer and robust observer, are respectively proposed. The observation error in the two schemes can be made arbitrarily small by choosing the appropriate design parameters. An illustrative example is provided to demonstrate the validity of the proposed design methods. (C) 2014 Elsevier Ltd. All rights reserved.