Embryonic stem cells are continuously growing stem cell lines of embryonic origin. The distinguishing features of ES cells are their capacity to be maintained in an undifferentiated state indefinitely in culture and their potential to develop into various kinds of cells in the body. At molecular level, in vivo and in vitro differentiation of ES cells are similar in many ways. To date, ES cells can differentiate in vitro into myocytes and myocardial cells, neural precursor cells, hematopoietic cells and endothelial cells. Recent evidence suggests that hematopoietic stem cells (HSCs) originate from a region within the embryo proper. It is likely that the genesis of HSCs is regulated by embryonic growth factors, which have not been discovered yet. ES cells have provided a convenient and effective experimental approach to define early hematopoietic process within embryos. Research on ES cells has broad application prospect.