Various types of electromagnetic radiation can be stimulated by free electrons interact with the local electromagnetic environment. Metamaterial is a kind of artificial material, which can achieve unique characteristics that natural material cannot realize. Based on the interaction between the metamaterial and free electron, the radiation characteristics can be flexibly manipulated such as polarization, phase and wavefront, and so on. The related study provides a novel platform for developing novel free-electron radiation devices. This paper briefly introduces the physical mechanism of Cherenkov radiation and Smith-Purcell radiation. Then, the state-of-art of free-electron radiation in metamaterials is introduced. Finally, an outlook of potential research directions for this vigorous realm is provided.