Oxygen fugacity (f O₂), a quantitative factor in rendering the redox state of a given system, is a key thermodynamic parameter in Earth sciences. The oxygen fugacity of the early Earth and its variation with time, which is a prime goal of Earth sciences, plays important roles in understanding the origin and evolution of the atmosphere, hydrosphere, biosphere and even the whole Earth. Zircon provides the oldest samples yet found on the planet, and is almost the only survivor of the Hadean period. Studies in recent years have demonstrated that the Ce content of zircon is able to record the prevailing f O₂ of its parent magma, leading to the so-called Ce-in-zircon oxybarometer. This provides a unique probe into the redox state of the early Earth. We provide a brief introduction of this technique, and then outline the recent advances on the redox state of the early crust and mantle, followed by a discussion on the f O2 evolution of the early atmosphere, crust and upper mantle and the coupling relation of f O₂ between them.