Abstract： Objective To study the cytothesis effects of haemopoietic stem cells (HSCs) from rat fetal liver on brain injury induced by radiation. Methods Twenty-four female SD rats were randomly divided into normal control group, model group and treatment group (n=8). Total brain irradiation with 20 Gy X-ray was performed in the model group and the treatment group. Rats in the treatment group received infusion of HSCs from fetal liver (3×106 cells) 24 h, and 7, 14 and 21 d after irradiation, while rats in the model group and controls received infusion of saline at the same dosage. The serum of the rats was collected 12 h after infusion of HSCs each time; spectrophotometry was employed to detect the levels of malonaldehyde (MDA) and superoxide dismutase (SOD); the contents of tumor necrosis factor α (TNFα), interleukin (IL-1 and IL6) were measured by ELISA. Rats were sacrificed on the 28th d for assessing the pathological changes in brain tissue by HE staining and the mRNA level of Y chromosome by RT-PCR. Results Compared with that in the model group, the level of SOD in the treatment group was significantly higher, but the levels ofMDA, TNFα, IL-1 and IL6 were significantly lower in the treatment group 24 h, and 7, 14 and 21 d after the irradiation (P＜0.05). The mRNA expression of Y chromosome was noted and apoptosis of Nissl bodies was also observed in the treatment group; the cerebral cortex in the treatment group was significantly thicker as compared with that in the model group (P＜0.05); regenerative hippocampal neurons were observed in the treatment group. Conclusion HSCs from fetal liver can effectively repair the radiation brain injury.