Abstract: The frequency characteristics of a hub-beam system are investigated in this paper. This rigid-flexible coupling system consists of a rigid hub, a flexible beam and a torsional spring. A finite-element model for this system is derived and all the couplings between the system rigid and flexible degrees of freedom are included. The frequency characteristics of this system are related to two ratios: the ratio J' of the beam and rigid hub inertia, the ratio K' of the torsional spring and beam bending stiffness. The effects of two ratios on the system frequencies are analyzed by numerical simulation and experimental test, respectively. The experiment is conducted in a single-axis air-bearing testbed. Both the theoretical results and experimental results show that the two ratios, i.e. K' and J', have significant influences on the system frequency characteristics. The dominant frequency of this system in steady responses can transit between the rigid and flexible frequencies in case of different values of K' and J'.