Silver nanocrystals with uniform sizes were synthesized in droplet microreactors through seed-mediated growth. The key to the success of this synthesis is the use of air as a carrier phase to generate the droplets. The air not only separates the reaction solution into droplets but also provides O-2 for the generation of reducing agent (glycolaldehyde). It also serves as a buffer space for the diffusion of NO, which is formed in situ due to the oxidative etching of Ag nanocrystals with twin defects. For the first time, we were able to generate Ag nanocrystals with controlled sizes and shapes in continuous production by using droplet microreactors. For Ag nanocubes, their edge lengths could be readily controlled in the range of 30-100 nm by varying the reaction time, the amount of seeds, and the concentration of AgNO3 in the droplets. Furthermore, we demonstrated the synthesis of Ag octahedra in the droplet microreactors. We believe that the air-driven droplet generation device can be extended to other noble metals for the production of nanocrystals with controlled sizes and shapes.