SynaptotagminⅠ(sytⅠ) is an abundant integral membrane protein of synaptic vesicle and the C2AB domain is the important functional domain in its cytoplasmic part. Recent studies show that C2AB prefers to interact with plasmic membranes of neuron cells in vivo and it is believed that such interaction is closely related to the sytⅠ physiological function as a Ca²⁺ sensor in the Ca²⁺-regulated neurotransmitter release, but the mechanism of the interaction is not clearly understood. Monolayers at an air/water interface combined CD and fluorescence experiments were used to study the characteristic of interaction between C2AB and a phospholipid membrane. The results in the monolayer experiment showed that C2AB domain preferred insertion into the negatively charged phosphatidylserine monolayer and Ca²⁺ ions were required for the interaction. Electrostatic force was mostly responsible for the insertion of C2AB into PS monolayers. Further CD and fluorescence experiments showed that the secondary structure of C2AB domain in the presence or absence of PS/PC liposome had some relatively small change. The experiments provide useful information concerning the important role of sytⅠ as a Ca²⁺ sensor in the fusion of secretary vesicles to the plasma membrane, and better understanding the mechanisms of membrane fusion in exocytosis.