Introduction: Considering the importance of cell migration in cell therapy protocols and the necessity to understand this issue better. This study was administred to evaluate the homing possibility of human umbilical cord blood mononuclear cells in the irradiated and non-irradiated rats’ bone marrow. Materials and Methods: Human umbilical cord blood was collected and after isolation and cell counting, 5 × 10 6 cells were injected to tail veins of 13 groups (1 healthy and 12 irradiated group) of adult rats. In defined time gaps after injection, transplanted rats were sacrificed and their bone marrow cells were collected. Then, the DNA of these collected cells was extracted and was amplified by PCR using specific primers for human STS. Finally, PCR products were analyzed on gel electrophoresis. Results: PCR results for irradiated and non-irradiated rats in all groups were negative which implied that human umbilical cord blood mononuclear cells did not home in bone marrow of transplanted rats. Conclusion: Our results suggest that human umbilical cord blood mononuclear cells are unable to home in rat's bone marrow after intra-venous injection. This might be the result of trapping of cells into other organs or unknown conditions of cells physiology for migration and homing.