In this paper, we generalize the concept of Hankel norm for purely continuous-time systems to sampled-data systems. The Hankel norm of a sampled-data system is defined by taking into account of intersample behaviors. The formula of the Hankel norm of a sampled-data system is given by a hybrid state-space model approach. We show that the Hankel norm is equal to that of an equivalent discrete-time system. Two simple examples including sampled-data controller reduction are demonstrated.