Abstract: Objective To determine the optimal time for MRI scanning of tumors in BALB/c mice using c-erbB2 antisense probe labeled with superparamagnetic iron oxide nanoparticles. Methods SK-Br-3 tumor-bearing BALB/c mice (n=30) were injected with antisense probe (12.0 mg Fe/kg). MRI scanning was performed on 5 mice before the injection and 60 min,180 min,360 min,720 min and 1 440 min after the injection, respectively. Tumor tissues were taken immediately after the scanning and fixed with 10% formalin and paraffin-embedded sections. The MRI signal strength of the tumors and adjacent muscles were compared with changes detected under a microscope using HE and Prussian blue staining. Results SK-Br-3 tumors were introduced to the BALB/c mice successfully. The strongest signal intensity was detected by the MRI 360 min after injection with the antisense probe. The pathological examination revealed structural disorders of the tumor issues, with a large number of special-shaped cells arranged in a cancer nest shape. Punctuate blue iron particles were observed in all of the tumor issues, with the greatest density occurring at 360 min after the injection with the antisense probe. Conclusion The optimal time for MRI scanning of tumors in BALB/c mice using c-erbB2 antisense probe labeled with superparamagnetic iron oxide nanoparticles should be set at 360 min after injection.