Silver nanoshells have wide-ranging applications spanning from catalysis to bio-sensing because of their unique optical and chemical properties. They are synthesized through a seed-mediated growth process, in which surfaces of the core particle are decorated with metal seeds followed by the growth of silver. However, a typical synthesis is a time-consuming batch-process which creates inhomogeneity in the shell structures. Hence, it is required to establish a quick, simple, and robust synthetic method. Using a central collision type microreactor instead of a batch reactor is advantageous because of its large surface to volume ratio and rapid mixing. In this study, we synthesize silver nanoshells by using such a microreactor and investigate the effects of the molar ratio of ammonia to silver ion, the types of reducing agents, and the addition of polyvinylpyrrolidone (PVP) on the complete nanoshell formation. Our experiments demonstrate that a large molar ratio of ammonia to silver ion is required to form silver-ammonia complexes and regulate the reducing power of reductants to cover the core particles with silver. Formaldehyde, due to its weaker reducing power, is found to be a more suitable reducing agent than ascorbic acid. PVP molecules serve not only as a stabilizer but also as a facilitator to help smooth shell growth. Most importantly, the use of the microreactor is critical to the formation of complete silver nanoshells.