Circulating microRNAs (miRNAs), also known as secretory miRNAs, are packaged in small membrane vesicles called exosomes. These exosomal miRNAs are secreted from various cell types and incorporated inside the recipient cells. The functions of exosomal miRNAs are poorly understood, but some reports have shown their essential roles in cancer development. Therefore, methods to study the function of exosomal miRNAs not only in vitro but also in vivo might be essential. We have analyzed the function of exosomal miRNAs by miRNA-enriched exosomes both in vitro and in vivo. In this chapter, the methods to concentrate the targeted miRNAs are provided. This simple and useful method enables the study of the precise mechanisms of exosomal miRNAs under physiological and pathological conditions.