Understanding the electrooxidation process of metal nanoparticles (NPs) is very important because they have showed wide applications in electrocatalysis, sensing, and nanoelectronics. In this letter, we designed a strategy to investigate the anodic stripping voltammetry (ASV) of silver oxidation at single NP level and aggregation state by using gold single nanoelectrodes (SNEs) and ultramicroelectrodes. Results showed that the ASV peak potential and shape were significantly affected by the diameter and aggregation degree of the Ag NPs: symmetrical-peak shape, two-peak shape, and asymmetrical-peak shape appeared when Ag NPs changed from the single particle state to the large-aggregation state, and size-dependent ASVs for Ag NPs oxidation were also observed. These findings can help us deeply understand the metal NPs oxidation process and will benefit the related applications.