Mesenchymal stem cells (MSCs) have considerable therapeutic potential and attract increasing interest in the biomedical field. MSCs are originally isolated and characterized from bone marrow (BM), then acquired from tissues including adipose tissue, synovium, skin, dental pulp, and fetal appendages such as placenta, umbilical cord blood (UCB), and umbilical cord (UC). MSCs are a heterogeneous cell population with the capacity for (1) adherence to plastic in standard culture conditions, (2) surface marker expression of CD73+/CD90+/CD105+/CD45-/CD34-/CD14-/CD19-/HLA-DR- phenotypes, and (3) trilineage differentiation into adipocytes, osteocytes, and chondrocytes, as currently defined by the International Society for Cellular Therapy (ISCT). Although BM is the most widely used source of MSCs, the invasive nature of BM aspiration ethically limits its accessibility. Proliferation and differentiation capacity of MSCs obtained from BM generally decline with the age of the donor. In contrast, fetal MSCs obtained from UC have advantages such as vigorous proliferation and differentiation capacity. There is no ethical concern for UC sampling, as it is typically regarded as medical waste. Human UC starts to develop with continuing growth of the amniotic cavity at 4-8 weeks of gestation and keeps growing until reaching 50-60 cm in length, and it can be isolated during the whole newborn delivery period. To gain insight into the pathophysiology of intractable diseases, we have used UC-derived MSCs (UC-MSCs) from infants delivered at various gestational ages. In this protocol, we describe the isolation and characterization of UC-MSCs from fetuses/infants at 19-40 weeks of gestation.