Blood brain barrier (BBB) is a special cellular system that enables the nervous system to maintain in brains homeostasis by allowing the essential materials to pass while denying harmful substances. All of the drugs with high molecular weight and 98% of the drugs with low molecular weight cannot pass through the BBB. Only a few high liposoluble molecules, which have smaller than 500 Da molecular weight, can pass through the BBB. Thus, a great number of central nervous system diseases do not respond to this kind of drugs. However, in order to effectively treat a great quantity of neurological diseases there is increasing necessity for drugs which can pass through the central nervous system. In conclusion, in recent years, studies aimed new strategies for the passage of the drugs with high molecular weight through the brain have been carried out.