In this paper, we discuss the structural vibration controllability of flexible manipulators. In the spatial flexible manipulators, the structural vibration controllability would be configuration-dependent. Therefore, the flexible manipulator might have some vibration uncontrollable configurations and thus a discussion on the vibration controllability is a must. In order to understand the physical interpretation of vibration uncontrollable configurations we propose the `modal accessibility' concept which indicates how well the actuators can affect the structural vibration modes. The configuration in which all actuators can't affect at least one of the vibration modes of the manipulator is vibration uncontrollable. To clarify this point, we consider an example and perform experiments on vibration suppression control of the flexible manipulator. Experimental results clarify the modal accessibility concept.