BACKGROUND: Olfactory ensheathing cells promote axonal regeneration, provide nutritional support for the injured host cells and regulate inflammation reaction, which possess potential for spinal cord injury repair. OBJECTIVE: To explore the optimal time window for intravenous transplantation of olfactory ensheathing cells in the treatment of spinal cord injury. METHODS: Thirty male SPF level rats were used to establish the rat models of spinal cord injury by spinal cord hemisection. Rat models were then randomly divided into five groups: 1-, 3-, 7- and 10-day olfactory ensheathing cell transplantation and PBS groups. Olfactory ensheathing cells were labeled with fluorescent quantum dots. PBS was injected into the rats in the PBS group after spinal cord injury. The injured spinal cord was removed at 1 day after injection. A small animal imager was used to measure the fluorescence transferred to the lesion at different time points. The number of cells transferred to the lesion was measured based on the intensity of fluorescence. The Anti-p75 NGF Receptor antibody was used for immunohistochemistry detection of the injured spinal cord. The study was approved by the Ethics Committee of Animal Laboratory of Ningxia Medical University, No. 2017-073. RESULTS AND CONCLUSION: Fluorescent quantum dots could label olfactory ensheathing cells. Results of fluorescence assay and immunohistochemistry indicated that transplanted olfactory ensheathing cells were transferred to the lesion at 1, 3, 7 and 10 days. Most cells were transferred to the lesion at 7 days. Therefore, these results indicate that olfactory ensheathing cells transplanted at different time points after spinal cord injury can be transferred to the lesion, with a number peak at 7 days that is the best time window for cell transplantation. [ABSTRACT FROM AUTHOR]