Polymerization-induced self-assembly (PISA) is regarded as one of the most important strategies in macromolecular nanotechnology, as it can create a wide range of nanoparticles at high concentrations and on a large scale. However, open-to-air PISA with biodegradable product is still a complicated challenge, as traditional PISA is usually carried out under oxygen-free conditions to afford nonbiodegradable polymers. To meet the above challenges, we propose a convenient one-pot open-to-air ring-opening polymerization (ROP) of N-carboxyanhydride (NCA)-induced self-assembly (NCA-PISA) at 10 °C, without the need for degassing, heating, catalysts, or chain transfer agents. The morphologies of nanoparticles depend on the ratio of the initiator to the monomer and the solid content. Polymer vesicles can be fabricated when the ratio and the solid content are 1:20 and 20%, respectively. Overall, this versatile one-pot NCA-PISA provides an insight into facilely fabricating biodegradable nanoparticles in air.