Abstract: Expansin is first identified as cell-wall-loosening protein. Its function is involved in regulating cell expansion and fruits softening. Because its typical characterization is the ability to induce the extension of isolated cell walls, extensometer is necessary for detection activity of the protein. A new sensitive and accurate extensometer with simple structure was installed using transducer as the key component. Characteristics of expansins from roots, stems, epicotyls and mature leaves of Vicia faba were studied using the extensometer. Results showed that there were expansins in all the tested organs and it was approved further by western-blot analysis. The activity of expansins from roots and leaves was higher than that from epicotyls and stems. The results suggested that our extensometer was sensitive enough to study expansins and the existence of expansin in mature leaves was reported firstly.