In general, the higher the post-transplant chimerism, the fewer relapses and the better the prognosis. Stem cell transplantation is a common treatment for leukemia. Chimerism refers to the phenomenon that donor and recipient cells can “coexist” in the body after transplantation of donor cells into the recipient. Chimerism is often used to assess the degree of chimerism between donor and recipient cells after stem cell transplantation, and can be calculated by measuring the percentage of donor cells in the recipient after transplantation. In particular, when the percentage of donor cells in the recipient’s bone marrow or peripheral blood is greater than 99%, it is called a state of complete donor chimerism, which is often indicative of a good prognosis. Chimerism testing after transplantation is important in predicting the relapse rate and survival of leukemia and requires close attention. Specific treatment should be directed by a physician.