Abstract: Metamaterials are artificial sub-wavelength structures with unique physical properties that cannot or cannot easily be realized using natural materials. These unique physical properties include negative refractive index, reversed Doppler effect, reversed Cherenkov radiation, and enhanced transition radiation, which mainly depend on the shape, size, and arrangement of metamaterial unit cells. When the charged particles interact with the metamaterial, the novel reversed Cherenkov radiation or enhanced transition radiation is excited, which coincides with the operating principle of vacuum electron devices. Based on the reversed Cherenkov radiation or enhanced transition radiation, and combined with the strong resonance and sub-wavelength characteristics of metamaterials, a series of metamaterial-inspired vacuum electron devices with significant miniaturization and high efficiency have been developed. The metamaterial-inspired vacuum electron devices based on the reversed Cherenkov radiation have reversed Cherenkov radiation oscillator and reversed Cherenkov radiation amplifier. The measured electron efficiency of the reversed Cherenkov radiation oscillator is as high as 19.54%, and the diameter of the metamaterial slow-wave structure is only 0.33λ （λ is the wavelength in the free space）. The metamaterial-inspired vacuum electron devices based on the enhanced transition radiation include the metamaterial extended interaction oscillator, the metamaterial extended interaction klystron, and the metamaterial klystron. The volume of the metamaterial klystron is about 0.44 of the conventional klystron, and the measured electron efficiency is 57.4%. The above experimental results confirm the advantages of miniaturization and high efficiency of the metamaterial-inspired vacuum electron devices, which will have important application prospects in large-scale scientific facilities, radar, communications, medical imaging, microwave heating, and so on.