The lithium nucleation and growth on a substrate during electroplating is a crucial phenomenon for the cycle life and safety of Li ion batteries (LIBs) or Li metal batteries (LMBs), in particular the interfacial instability of Li metal anode. Hence, understanding the nucleation and growth mechanism of Li metal deposition is an essential key for providing a long cycle life and safe LIBs or LMBs. However, no quantitative report on Li metal deposition is found to the best of our knowledge. In this study, we proposed for the first time a model for nucleation and growth mechanism of Li metal associated with SEI formation by a chronoamperometry method. The current transients at different applied potentials initiate the nucleation and growth of Li metal on Cu substrate. A model considering 3D diffusion-controlled instantaneous process with the simultaneous reduction of electrolyte decomposition due to the SEI fracture is first employed for investigating the nucleation and growth of Li metal. This proposed model opens a new way to quantitatively understand the nucleation and growth of Li metal and electrolyte decomposition in different electrolytes.