Abstract: Based on the intrinsic relations under uniaxial strain conditions, the effective shear modulus defined by Cochran and Guinan was correlated to the longitudinal and bulk sound velocities, and used to study the quasi-elastic behavior of LY12 Al. Unloading wave profile measurements using VISAR technique were performed on LY12 Al over the shock stress ranging from 20 to 100 GPa from which sound velocities and the effective shear modulus along the quasielastic release path were evaluated. Results show that the effective shear modulus decreases rapidly with the release stress and can be approximated by a linear function of release stress. The slopes of the linear function increase with the initial shock-loading stress which is implied that the quasi-elastic behaviors are depended on the shock-loading stress. By using the linear function of effective shear modulus, the performed numerical simulations well reproduce the release wave traces of the Al-alloys reported in the literature. The effective shear modulus in the present study is compared with that obtained from quasi-static experiments and the possible micromechanisms of quasi-elastic release behavior in shock-loaded materials are also discussed.