The blood-brain barrier (BBB) is a huge obstacle in therapy of brain diseases, for it hinders the delivery of water-soluble molecules and those with molecular weight above 500 from the circulation system to the brain. Polysorbate 80 (Tween 80, T-80)-coated polylactid acid(PLA) nanoparticles represent a tool to transport such drugs across the BBB. Transcytosis is put forward as one mechanism of drug-loaded nanoparticles across the blood-brain barrier (BBB). However little is known about it. Electron microscopy is an important method in the investigation on nanoparticles injected into the experimental mice. In this study it was found by fluorescence microscope that fluorescence existed along the capillary dissepiment. Some nanoparticles distributed in the brain capillary endothelial cells and brain tissue outside the microvaculum, which was observed by transmission electron microscopy. These particles were proved to be the Copper chlorophyll loaded nanoparticles which containing Cu detected by AEM. The in vivo experiments demonstrated directly that the PLA nanoparticles can pass the BBB indeed and transcytosis by microvascular endothelial cells may be the mechanism. The results provided an efficient way of drug-delivery targeting the brain. Copper chlorophyll could be used as a new symbol of nanoparticles in in vivo experiment.