Neurotransmitter release is a precisely orchestrated process in terms of time and space in neuron. SNAREs have been identified to function as the basic machinery mediating membrane fusion during neutotransmitter release. Many forms of neurodegeneration initiate presynaptically, but few of their molecular mechanisms have been revealed clearly. &agr;-Synuclein (&agr;-Syn) is a highly conserved synaptic vesicle-associated protein. Aggregation of &agr;-Syn is a major component of the Lewy bodies, which is characteristic of Parkinson's disease (PD). We studied the effect of &agr;-Syn on SNARE-mediated membrane fusion using fluorescent methods. Bulk lipid mixing assay shows that &agr;-Syn has a role of inhibition in fusion and this effect requires phosphatidylserine (PS) on the vesicles. Disease related &agr;-Syn mutants, A30P and E46K, shows higher inhibition effect on the lipid mixing than wild type. Synaptotagmin-1 (Syt-1) is a Ca²⁺ sensor localized to synaptic vesicles and regulates neuronal exocytosis. C2AB, a soluble model of Syt-1 that lacks the transmembrane region, is shown here to accelerate the FRET significantly. This acceleration effect of lipid mixing also needs PS on the vesicles. &agr;-Syn can inhibit C2AB's stimulatory effect to a large extent. Thus, &agr;-Syn can inhibit SNARE-mediated membrane fusion event.