Objective：This study aims to investigate the clinical significance of Tim3 in differential diagnosis of aplastic anemia（AA）and myelodysplastic syndrome（MDS）. Methods：The proportion of CD4+Tim1+ and CD4+Tim3+ cells in peripheral blood mononuclear cells（PBMC） from 24 AA patients，20 MDS patients，and 17 normal controls were detected by flow cytometry. Real time qPCR was performed to examine Tim3 mRNA and Gal-9 mRNA levels in PBMC. Results：There was no difference of the proportion of CD4+Tim1+ cells in PBSC among normal control，AA，and MDS groups；The proportion of CD4+Tim3+ cells in MDS patients was significantly higher than that in patients from AA and control groups（P < 0.05）；The percentage of CD4+Tim3+ cells was significantly increased in high-risk MDS patients compared with that in low-or medium-risk MDS patients（P < 0.01）and healthy controls（P < 0.05）. There was no difference of Tim3 mRNA and Gal-9 mRNA levels in PBSC among normal control group，AA group，and MDS group. Conclusion：Tim3 in MDS patients is significantly higher than that in AA patients and is related to the risk of disease. These findings suggest that Tim3 might be involved in the pathogenesis of MDS and might be utilized as a marker to distinguish AA and MDS.