Increasing demands for high performance of vibration control drives the development of nonlinear vibration control devices to overcome the limitations of traditional linear vibration control methods. A novel and compact X-structured mount (X-mount) with an oblique spring is designed and investigated in this paper which demonstrates beneficial nonlinear stiffness and damping, tunable payload and resonant frequency property, and a special enlarged quasi-zero-stiffness region, which thus lead to excellent vibration isolation performance without instability or other strong nonlinear response such as jump phenomenon. The X-mount is designed, based on a bio-inspired X-shaped structure, and a special oblique spring is introduced which can effectively widen the working quasi-zero-stiffness range by employing negative stiffness due to a special contact stiffness. The oblique spring can also help to reduce the dynamic stiffness achieving an ultra-low natural frequency and broadband effective vibration isolation range. The mathematical modeling, theoretical analysis and experiment results show the high performance of this X-mount, potentially providing a promising solution for vibration control in many engineering practices.