State analysis of time-varying singular nonlinear systems via Haar wavelets is proposed in this paper. Based upon some useful properties of Haar wavelets, a special product matrix and a related coefficient matrix are applied to the time-varying systems such that the state of time-varying singular nonlinear systems can be solved easily. The local property of Haar wavelets is advantageous to shorten the calculation process in the task. We finally give a brief comparison between Haar wavelet and other orthogonal functions. (C) 1999 IMACS/Elsevier Science B.V. All rights reserved.