Objective To observe the ultrastructure of blood-brain barrier in the nude mouse model of brain metastases from lung cancer by transmission electron microscopy using lanthanum nitrate tracing. Methods PC-9 cells (1×10⁶/0.1 mL) in logarithmic phase were respectively injected into six nude mice (model group) selected from eight nude mice randomly via the left ventricle, the other two mice without any treatment as the control group. The general status of the mice was observed after implantation. In the fourth week all the mice were sacrificed and brain tissue samples were taken and prepared for transmission electron microscopic observation using lanthanum nitrate tracing. besides, the lung and brain were removed and stained with HE to detect the presence of tumor metastasis. Results Mice in the model group began to lose weight almost simultaneously in the third week and became moribund slowly, and were all sacrificed at the fourth week when showing clear signs of cachexia. At autopsy, the thoraxes were clear, with normal lungs. Histology showed evidence of brain metastasis in all the six mice. The electron microscopy showed that lathanum nitrate tracer was escaped from the capillaries and diffusely or sparsely distributed in the brain tissues of the model group mice, however lathanum nitrate tracer was still confined in the capillary lumen in the mice of control group. Conclusions The diffuse lathanum nitrate tracer in the brain parenchymal tissue indicates the impairment of blood-brain barrier in the nude mouse model of lung cancer brain metastasis and the formation of these metastases is accompanied with the destruction of blood brain barrier.