Thrombopoietin (TPO) is a major cytokine for megakaryocytopoiesis and thrombopoiesis, and also plays an important role in the regulation of early hematopoiesis. TPO activates a number of signal pathways to exert its biological function by binding to its receptor (c-mpl). Once these signal pathways (including Jak/STAT, PI3K/Akt, Ras/MAPK) are activated, the expression of the downstream signal molecules can be changed, which then induces biological effects. Recent researches have suggested the novel functions of TPO in many systems. The receptor of TPO (c-mpl) has been shown not only present in hematological cells, but also in many other cells and organs, such as neurons, heart muscle cells, vessel endothelial cells and so on. TPO exerts a protective effect on these cells through the interaction with c-mpl. This review discusses the molecular mechanism of TPO signal and the effect of TPO on multi-nonhematopoietic cells.