Metallic lithium (Li) is considered as one of the highly interesting anode materials for advanced batteries due to its large theoretical capacity, small material density and the high cell voltages that can be obtained in batteries using Li anodes. However, Li dendrite growth and unstable solid electrolyte interphase (SEI) formation emerged during the plating-stripping process, leading to low coulombic efficiency, rapid battery degradation and serious safety issues. These disadvantages form the major challenges towards commercialization. In this work, an ultra-thin and uniform Y2O3 layer is coated on Li metal anodes by atomic layer deposition (ALD) for improving the stability of Li metal batteries. Elucidation by in vacuo X-ray photoelectron spectroscopy (XPS) revealed different growth of a Y2O3 layer on metallic Li compared to that on a silicon wafer, which is traditionally used for ALD processes. The Y-precursor, i.e., Y(EtCp)2(iPr-amd), firstly reacts with metallic Li and forms a "decomposition" layer, leading to nonlinear growth at the preliminary stage (up to around 20 ALD cycles). Only after the bare Li surface has been fully covered does standard ALD growth start. The ALD Y2O3 layer on metallic Li is able to effectively prevent the growth of Li dendrites, giving rise to an even plating-stripping process in symmetric Li metal cells with more stable performance and prolonged lifespan. Improved electrochemical performance through the Y2O3 protective layer has also been investigated in Y2O3-coated Li||LiMn2O4 asymmetrical full cells. This work indicates that ALD Y2O3 coating is an attractive method to stabilize Li metal anodes for battery devices.