A shear module plays an important role in modeling of soil behavior, especially in small strain structures such as vibration equipment foundation, wind turbine foundation, bridge foundation, retaining wall, tunnel, and so on. There are a lot of studies on soil shear modulus depending on the allowable shear strain in the world, but there is not much that study in Vietnam, especially in small strain structures. In this paper, to study the relationship between the shear modulus and the allowable shear strain, the authors used downhole seismic tests, cycle triaxial tests and standard penetration tests performed on coastal sand in Quang Binh province, Vietnam. The results from the shear modulus are compared with those in the classical literature to enrich the database of the shear modulus for engineering practices in Vietnam. The experimental results from the shear modulus show that the correlation between the shear modulus and the allowable shear strain is close to the lower limitation of the classical correlation curve in the literature.