Abstract： Objective To investigate the value of the multiple short tandem repeat (STR)amplification by fluorescence labeling polymerase chain reaction (PCR) combined with fusion gene bcr-abl mRNA expression for quantitative determination of chimerism and qualitative detection of bcr-abl transcripts,and to evaluate the status of engraftment and predict the outcome of allogeneic hematopoietic stem cell transplantation (allo-HSCT). Methods 5 relapse patients with CML after alIo-HSCT were dynamically investigated. Quantitative analysis of donor chimerism was performed by multiplex PCR amplification of STR markers and capillary electrophoresis with fluorescence detection, qualitative detection of bcr-abl transcripts was performed by RT-PCR. Results The donors alleles appeared in all of 5 patients on day 28 post transplant, and bcr-abl expression was negative. But 5 patients had unstable mixed ehimerism. (DC: 0～80.4 %) at the different time points after aIIo-HSCT and bcr-abl was positive. One of them kept eontinuely the mixed chimerism in the relapse of disease, and died after one year, and the other changed from MC to CC by intervention of clinical treatment. Reduction of donor chimerism were detected prior to the occurrence of graft rejection and disease relapse, while bcr-abl gene expression was positive. Conclusion The results of STR-PCR in the range of its sensitivity fully correspond with bcr-abl tests in patients with CML. The combination of STR-PCR with RT-PCR provides a highly sensitive and valuable tool for engraftment evaluation, graft rejection, relapse and predicting GVHD. Furthermore it can provide a basis for early intervention of clinical treatment, and can identify these patients at high risk with molecular or cytogenetic relapse after allo-HSCT.