Traditional cooling systems consume tremendous amounts of energy and thus aggravate the greenhouse effect1,2. Passive radiative cooling, dissipating an object's heat through an atmospheric transparency window (8-13 μm) to outer space without any energy consumption, has attracted much attention3-9. The unique feature of radiative cooling lies in the high emissivity in the atmospheric transparency window through which heat can be dissipated to the universe. Therefore, for achieving high cooling performance, the design and fabrication of selective emitters, with emission strongly dominant in the transparency window, is of essential importance, as such spectral selection suppresses parasitic absorption from the surrounding thermal radiation. Recently, various materials and structures with tailored spectrum responses have been investigated to achieve the effect of daytime radiative cooling6-8,10-15. However, most of the radiative cooling materials reported possess broad-band absorption/emission covering the whole mid-infrared wavelength11-15. Here we demonstrate that a hierarchically designed polymer nanofibre-based film, produced by a scalable electrostatic spinning process, enables selective mid-infrared emission, effective sunlight reflection and therefore excellent all-day radiative cooling performance. Specifically, the C-O-C (1,260-1,110 cm-1) and C-OH (1,239-1,030 cm-1) bonding endows the selective emissivity of 78% in 8-13 μm wavelength range, and the design of nanofibres with a controlled diameter allows for a high reflectivity of 96.3% in 0.3-2.5 μm wavelength range. As a result, we observe ~3 °C cooling improvement of this selective thermal emitter as compared to that of a non-selective emitter at night, and 5 °C sub-ambient cooling under sunlight. The impact of this hierarchically designed selective thermal emitter on alleviating global warming and temperature regulating an Earth-like planet is also analysed, with a significant advantage demonstrated. With its excellent cooling performance and a scalable process, this hierarchically designed selective thermal emitter opens a new pathway towards large-scale applications of all-day radiative cooling materials.