Abstract. This paper discusses balanced realization and model order reduction for both continuous-time and discrete-time general nonlinear systems based on singular value analysis of the corresponding Hankel operators. Singular value analysis clarifies the gain structure of a given nonlinear operator. Here it is proved that singular value analysis of smooth Hankel operators defined on Hilbert spaces can be characterized by simple equations in terms of their states. A balanced realization and model order reduction procedure is derived based on it, and several important properties such as stability, balanced form, Hankel norm, controllability, and observability of the original system are preserved. 1. Introduction. In the theory of stable linear systems, the system Hankel operator plays an important role in a number of problems. Its relation to the state-space concept of balanced realizations, where the Hankel singular values are important, is well understood nowadays [32] and provides a powerful tool for model reduction of linear control systems. In this paper we propose a framework for a general class of stable nonlinear systems, where balanced realizations are directly related to the Hankel operator of the nonlinear system. This in turn provides a tool for model reduction of the nonlinear system, where properties such as stability, the Hankel norm, and the balanced form of the system are preserved. Our approach builds further upon the earlier developments in [6].A first nonlinear extension of the linear state-space concept of balanced realizations has been introduced in [24], mainly based on studying the past input energy and the future output energy. Since then, many results on nonlinear state-space balancing, related minimality considerations, balancing near invariant manifolds, computational issues for model reduction, flow balancing, trajectory piecewise linear balancing, and empirical balancing for nonlinear systems have appeared in the literature; see, e.g., [7,8,10,11,12,14,16,21,23,25,27,28,30,31].In our earlier work, the relation of the state-space notion of balancing for finite dimensional, continuous-time, input affine nonlinear systems with the nonlinear Hankel operator has been considered; see, e.g., [11,25,26]. In particular, the singular value functions of [24], which can be viewed as a nonlinear state-space extension