Thin-film lithium niobate (TFLN) micro/nano-structures allowing strong light field confinement to boost the interaction between light and matter, shows great potentials for cutting-edge researchs and high-tech applications for high speed optical communications, high-performance optical computation, integrated photonics, and integrated quantum information processing, etc. This article reviews the basic principles, manufacturing technical advances, and the latest progress of TFLN microdevices based on microresonators and waveguides, including various applications ranging from frequency conversion, electro-optic modulation, optical frequency combs to microwave-to-optical wave conversion, quantum photonics, and microlasers. The application of these techniques has greatly promoted the development of large-scale integrated optoelectronic circuits and quantum networks. In addition, this article gives a perspective on potential applications in the future.