Thermal radiation, as a ubiquitous physical phenomenon, plays an important role in various research fields of science and engineering. Traditional understanding of thermal radiation mainly relies on Planck’s law, which describes the energy exchanging efficiency of entire thermal radiation process. However, recent studies indicated that comparing with the macroscopic object obeying Planck’s law, the thermal radiation in nanophotonic structures is obviously abnormal. This is due to the fact that the nanostructures’ featured size or neighboring space are much smaller than the thermal wavelength. It is important to notice that by well designing the material, size, and structure pattern, the thermal radiation is tunable and controllable. Furthermore, the nanophotonic structures enabling the radiative cooling effects promise to possess the tremendous applications including energy, ecology, etc. In this review paper, firstly, we briefly describe the fundamental theory of thermal radiation, as well as the history and latest progress, such as, enhanced radiative heat transfer, the near-field radiation in two-dimensional materials, and the overall far-field enhancement. Secondly, we focus on the newly available daytime radiative cooling system, which is based on metamaterials or desired nanophotonic structures, pursuing the best cooling performances. Finally, we detail the checklists of remarkable applications, ranging from building cooling and dew collection to solar cell cooling. In addition, we also point out the broad future of radiation cooling technology of nanometer optical materials in promoting the management and transformation of desert ecological environment.