Objective：To investigate the effect of external magnetic field on MR imaging of ovarian tumor in nude mice bearing tumor in vivo by SPIO-shRNA molecular probe. Methods：The concentration of probe injection was set as iron 18 mg/kg，and the scanning time was 27 h after probe injection. BALB/c mice of 30 were randomly divided into 3 groups，namely magnetic transfection group （experimental group），SPIO-shRNA group（negative control group），nude mice bearing cancer group（control group）. Sample capacity of each group was 10 and the experimental group was magnetic transfected before scan. Changes of signal intensity in tumor tissue and adjacent muscle tissue were scanned by magnetic resonance imaging（MRI）. The nude mice were killed after that treatment，tumor and muscle tissues were sampled for HE and prussian blue stain and observed under optical microscope. Results：The difference of tumor tissue signal intensity of T1WI sequence among three groups was not statistically significant（F=2.623，P=0.091） and the differ－ence of muscle tissue signal intensity in each group was not statistically significant（F=0.434，P=0.652）. Compared with those in the control group，the tumor tissue signal intensity of T2WI and T2* sequence in the experimental group and the negative control group were decreased，with statistically significant differences（P=0.000），and compared with that in the negative control group，it was more significantly in experimental group with statistically significant differences（P=0.000）. Compared with that in the other groups，the T2* sequence of muscle tissue signal intensity in the experimental group was relatively lower，with statistically significant difference（P=0.000）. HE staining showed that structure of tumor tissue was abnormal and nuclear atypia was increased；prussian blue staining showed that blue-stained iron particles existed in both experimental group and negative control group，with more in the experimental group. Conclusion：The external magnetic field can induce SPI－O-shRNA molecular probes target to ovarian tumor tissues and increase the effective concentration of tumor tissue absorption.