Abstract Background Intracellular reactive oxygen species (ROS) have dual effects depending on their cellular level. ROS act as secondary messengers at a low concentration, although ROS exhaust the hematopoietic stem cell (HSC) compartment at a higher oxidized state. So, we investigated whether maintaining a low level of ROS could preserve the hematopoietic stem cell function according to the MnSOD over expression. Methods Human MnSOD cDNA was introdu ced into mouse HSCs and progenitor cells by using a MSCV-PGK-GFP retrovirus. The hematopoietic function of over-expressing MnSOD was evaluated in vitro on a colony-forming cell assay and in vivo in a competitive transplantation model. MnSOD-transduced, lineage negative, GFP+ B6.SJL (CD45.1+) mouse bone marrow cells were transplanted into lethally irradiated C57BL/6J (CD45.2+) mice in competition with CD45.1/45.2 double positive bone marrow mononuclear cells. We also measured the basal mRNA levels of antioxidants, including MnSOD, catalase and cellular glutathione peroxidase (GPx1), of C57BL/6J HSCs. Results On the colony-forming cell assay, a MnSOD over expression significantly preserved the CFU-M with irradiation as compared with the mice without irradiation. HSCs with an MnSOD over expression showed a tendency for higher engraftment ability on the competitive transplantation assay even after 200 cGy re-irradiation, and we observed a significantly higher myeloid differentiation potential after the second serial transplantation. The basal mRNA levels of MnSOD and catalase were less than 1∼2% and 2∼5%, respectively, in the long-term and short-term HSCs, respectively, and these cells didn't activate in spite of radiation stress. Conclusion These results show that only an over expression of MnSOD without downstream catalase activation can not augment the mouse hematopoietic stem cell repopulation activity.