Abstract: Membrane fluidity means the fluidity of membrane lipid in the unit membrane. The changes of neuronal membrane potential, ion flux, release and binding of neurotransmitters, enzyme activity and signal transfer will induce the changes of neuronal membrane fluidity. The membrane fluidity of brain neurons of aged animals and Alzheimer s patients is lower than that of young animals and normal people. The decrease of membrane fluidity is related to the increase of lipid peroxidation induced by increased free radicals. In addition, the disorder of intracellular calcium homeostasis, decrease of density of membrane receptor, and deposit of amyloid-β protein (Aβ) in the neurons of the brain also affect the membrane fluidity. The nootropic drugs, such as Pyrrolidone derivatives, Ginsenoside Rg1 and EGb761 significantly improved membrane fluidity of neurons in the aged animal and Alzheimer s disease patient s brain via above mechanisms.